Der Iran hat der US-Regierung mit dem Ausstieg aus dem Wiener Atomabkommen gedroht, falls Washington dessen Umsetzung weiterhin blockieren sollte. "Wir sind auch auf diese Option vorbereitet", sagte Außenminister Mohammed Jawad Zarif am Dienstag. Der Iran könne "jederzeit zu dem Status von vor dem Abkommen zurückkehren", erklärte Zarif laut Nachrichtenagenutur ISNA weiter.

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Das Atomabkommen wurde im Juli 2015 vom Iran und den fünf UN-Vetomächten sowie Deutschland ratifiziert. Es räumt Teheran das Recht auf friedliche Nutzung der Kernkraft ein, aber nicht den Bau von Kernwaffen. Zudem sah es den Abbau gegen Teheran verhängter Wirtschaftssanktionen vor, doch halten die USA diverse Sanktionen aufrecht.

Irans oberster Führer Ayatollah Ali Khamenei unterstrich die Drohung indirekt. "Wenn unsere jungen Wissenschafter in kurzer Zeit Uran von 3,5 auf 20 Prozent anreichern können, wären sie auch in der Lage, es auf 99 Prozent anzureichern", sagte er laut der Nachrichtenagentur Tasnim. Über 90 Prozent angereichertes Uran kann auch für den Bau von Atombomben genutzt werden.

Bisher wurde das Atomabkommen nicht umgesetzt wie vom Iran erhofft. Schuld dran sind laut Teheran die USA, die besonders Wirtschaftsprojekte mit Europa sabotieren. Über lukrativen Verträge mit europäischen Unternehmen hoffte der Iran auf zahlreiche neue Arbeitsplätze im Land. Doch europäische Großbanken verweigern oft aus Angst vor amerikanischen Strafen die Finanzierung der Projekte. Daher steckt der Iran auch nach dem Abkommen noch in der Wirtschaftskrise.</s>After more than two weeks of negotiations in Vienna, Iranian and Western officials said the earliest an agreement could be ready was Monday, the self-imposed deadline for clinching a deal, though that could be extended again.

"We still have got work to do tomorrow," Iranian Foreign Minister Mohammad Javad Zarif told reporters from his hotel balcony. "No deal today."

U.S. Secretary of State John Kerry has cautioned that "major issues" remain to be resolved, and comments from both senior Republican and Democrat Senators on Sunday suggested that any final deal would also face tough scrutiny in Congress.

Alireza Miryousefi, a spokesman for the Iranian delegation, said on Twitter that the draft agreement Iran and the powers - the United States, France, Britain, Germany, Russia and China - were working on was "a 100-page document".

Western powers have long suspected Iran of aiming to build nuclear bombs and using its civilian atomic energy program to cloak its intention - an accusation Iran strongly denies.

The goal of the deal being negotiated would be to increase the time it takes for Iran to produce enough enriched uranium fuel for a single weapon to at least one year from current estimates of two to three months - the "breakout" time.

Iranian President Hassan Rouhani said on Sunday further measures were still needed to overcome the remaining obstacles in the Vienna negotiations, ISNA news agency reported.

"It might seem we have reached the top of the mountain. But no, there are still steps needed to be taken," ISNA quoted him as saying. "Even if we fail ... we have performed our duty."

In a sign that prospects for a deal being concluded on Sunday were fading, British Foreign Secretary Philip Hammond headed back to London but was expected to return on Monday, officials said.

French Foreign Minister Laurent Fabius canceled a planned trip to Africa due to the continued deadlock in the Iran negotiations, a French diplomatic source said.

"I hope, I hope, that we are finally entering the final phase of this marathon negotiation," Fabius told reporters.

A German government source said a deal could be reached quickly if Tehran was ready to take the last steps, adding that it was still possible for the negotiations to fail.

"Everything can still fail, but we are indeed very close to reaching the goal... Negotiations are currently in the absolute final phase and are running intensely through the night," said the source, speaking on condition of anonymity.

A senior Iranian official said 99 percent of the issues had been resolved, adding: "With political will we can finish the work late tonight and announce it tomorrow."

Among the biggest sticking points in the past week has been Iran's insistence that a United Nations Security Council arms embargo and ban on its ballistic missile program dating from 2006 be lifted immediately if an agreement is reached.

Russia, which sells weapons to Iran, has publicly supported Tehran on the issue.

Other problematic issues in the talks are access for inspectors to military sites in Iran, answers from Tehran over past activity and the overall speed of sanctions relief.

Kerry and Zarif have met nearly every day since Kerry arrived in Vienna more than two weeks ago for what was intended to be the final phase in a negotiation process that began with an interim nuclear deal clinched in November 2013.

Kerry met Zarif again on Sunday, a U.S. official said, in an attempt to break the deadlock.

Experts and senior officials from Iran, the United States and the other powers have been meeting non-stop for months, often working into the early hours of the morning, to finalize an accord that will include five technical annexes.

An agreement would be the biggest step towards rapprochement between Iran and the West since the 1979 Islamic Revolution, although both sides are likely to remain wary of each other even if a deal is concluded.

Iranian Supreme Leader Ali Khamenei said Tehran would continue its fight against "global arrogance" - referring to the United States. But Khamenei did not set any new "red lines" for his negotiators as he did in a tough speech two weeks ago.

In Washington, the top Republican in the U.S. Senate cast doubt on whether President Barack Obama would be able to win approval in Congress for any deal.

"I think it's going to be a very hard sell, if it's completed, in Congress," Senate Majority Leader Mitch McConnell said on the "Fox News Sunday" broadcast. "We already know it's going to leave Iran as a threshold nuclear state."

"There is no trust when it comes to Iran," said Senator Ben Cardin, the senior Democrat on the Senate Foreign Relations Committee that would examine any deal as part of the U.S. Congress' 60-day review.</s>Iran is in talks with the United States and five other powers - Britain, China, France, Germany and Russia - on an agreement under which it would curtail its nuclear program in exchange for relief from economic sanctions.

French Foreign Minister Laurent Fabius said some progress had been made and that he would return to Vienna on Sunday evening in the hope of clinching a final deal to end a 12-year standoff between Iran and the West.

Chinese Foreign Minister Wang Yi said there was a "high possibility" Iran and the six would succeed in reaching an accord in the coming days, though he cautioned that there were still difficult issues to resolve.

"We have confidence that finally the parties concerned will arrive at a fair, balanced and just solution," he said through an interpreter.

He added that they were "faced with some important and sensitive issues which no one can shy away from."

A senior Iranian official told reporters Iran and the six were discussing possible dates when key U.S., European Union and United Nations sanctions would be lifted and limits on Iranian nuclear activities put into effect.

The idea was to have both sides prepare everything so that the implementation of a deal would start on a specific day, he added. However, the official highlighted a major difference between Iran and Western powers when he said Tehran expected key international sanctions would be "terminated".

U.S. and EU officials have spoken of sanctions being suspended rather than ended, so that they can be reimposed if Iranian fails to comply with a deal.

Western powers and their allies suspect Iran is using a civilian nuclear energy program as a cover to develop a nuclear weapons capability. Iran denies this, saying its program is for peaceful purposes.

The negotiators missed a June 30 deadline for a final agreement but have given themselves another week, until July 7.

A deal would be a major policy achievement for both U.S. President Barack Obama and Iranian President Hassan Rouhani. Both face scepticism from powerful hardliners at home in countries that have been enemies since Iranian revolutionaries stormed the U.S. Embassy in Tehran in 1979.

Iranian Foreign Minister Mohammad Javad Zarif and U.S. Secretary of State John Kerry, who have been holding intense talks in Vienna, were joined on Thursday by the foreign ministers of Britain, Germany and China, as well as France.

Britain's Philip Hammond told reporters on arrival: "I don't think we're at any kind of breakthrough moment yet and we will do whatever we need to do to keep the momentum."

Iranian negotiator Majid Takhteravanchi told reporters: "We have reached the final days, but still it is not clear when the last day will be."

Major issues yet to be agreed also include monitoring and verification steps to ensure that Iran does not cheat on any agreement.

The powers want Iran to grant more access to inspectors from the International Atomic Energy Agency, and to answer its questions about previous nuclear work that may have had military purposes. IAEA chief Yukiya Amano was in Tehran on Thursday to meet Iranian officials.

Iran's Supreme Leader Ayatollah Ali Khamenei, who has the power to block a deal, last week ruled out either a long freeze of sensitive nuclear work or opening military sites to inspectors.

A Western diplomat told Reuters: "The positions set out by Khamenei last week make it more difficult to bridge the gaps in the next few days, and there is still work to be done."

In an apparent response to Khamenei, Obama said on Tuesday no deal would be agreed unless it blocked all Iranian pathways to developing a nuclear bomb and ensured a robust monitoring system was in place.

The senior Iranian official in Vienna said Iran would sign up to a special IAEA inspection regime called the Additional Protocol, which would be provisionally implemented at the start of a deal and, eventually, ratified by Iran's parliament.

The Protocol allows IAEA inspectors increased access to sites where they suspect nuclear activity is taking place, but U.S. officials say it is insufficient because it could allow Iran to stall by dragging out negotiations over access requests.

Iran's IAEA ambassador Reza Najafi called Amano’s visit "constructive", according to the state news agency IRNA. The IAEA had no immediate readout.

A senior U.S. official said this week the powers had agreed on a procedure to ensure the IAEA got the access it needed, though there was no suggestion that Iran had accepted it.</s>Ayatollah Ali Khamenei called on Iran's Revolutionary Guards to mass produce missiles and said the nuclear negotiations were not the place to discuss Tehran's defence programme or to solve the problem of sanctions damaging the Iranian economy.

"They expect us to limit our missile programme while they constantly threaten Iran with military action," Khamenei was quoted as telling the IRNA news agency while on a visit to an aeronautics fair held by the Revolutionary Guards.

"So this is a stupid, idiotic expectation ... The revolutionary guards should definitely carry out their programme and not be satisfied with the present level. They should mass produce. This is a main duty of all military officials."

Iran and the United States, France, Germany, Britain, China and Russia will reconvene in Vienna on Tuesday to try to iron out differences over how to end a long standoff over suspicions that Tehran has sought the means to develop nuclear weapons.

While missiles are not at the heart of the talks over Iran's nuclear work, which centre on the production of fissile material usable in atomic bombs, Western countries would like them discussed at the nuclear discussions.

Iran has one of the biggest missile programmes in the Middle East, viewing it as an essential precautionary defence against the United States and other adversaries such as Israel.

The United States and its allies fret that such missiles could potentially carry nuclear warheads.

The Islamic Republic denies accusations that it is seeking a capability to make nuclear weapons. It insists that the missiles are part of its conventional armed forces and rules out including them on the agenda of the nuclear discussions.

Khamenei's comments appeared at odds with more conciliatory remarks earlier by President Hassan Rouhani, who said he wanted Iran to do a better job of explaining its nuclear programme to prevent "evil-minded" people misleading world opinion.

"What we can offer the world is greater transparency," Rouhani said in a speech at a ceremony celebrating Iran's scientific achievements. Western powers have long demanded greater openness from Iran to address their concerns and head off a downward spiral towards a new Middle East war, with Israel threatening to attack its arch-foe if diplomacy does not succeed. Rouhani is a relative moderate compared to his predecessor, conservative hardliner Mahmoud Ahmadinejad, who used strident rhetoric during his eight years in office, serving to shore up international resolve to curb Iran’s nuclear programme.

Khamenei, who has the last word on all matters in Iran, has backed Rouhani's cautious opening to the West and nuclear talks with world powers, but has also said he doubts the West is ready to bury what he sees as its hostility to Tehran.

On Sunday, he expressed further such misgivings.

"I have always been in favour of initiative and negotiation in foreign policy ... but one must not tie our country's needs and issues like sanctions to the (nuclear) talks,” IRNA quoted Khamenei as saying. "Officials should tackle the sanctions issue in another way."

Iran in January halted its most sensitive nuclear operations under a preliminary deal with world powers, winning some relief from painful economic sanctions that have damaged its oil-dependent economy by forcing a sharp reduction in crude exports.

Rouhani said Iran if it so chose could resume enrichment of uranium gas to a fissile purity of 20 percent - its most sensitive nuclear activity because it is a relatively short technical step away from the level required for nuclear weapons.

"We wanted to tell the world that our activities are moving in the right direction: If we say we can enrich to 3.5 percent, we can do it. If necessary we will do (it to) 20 percent," he said.

Iran agreed under the Nov. 24 deal to shelve enrichment to 20 percent. It has since diluted some of its 20 percent-enriched stockpile to a lower concentration and converted some into an oxide less suited to processing into bomb-grade material.

Iran has justified its 20 percent enrichment drive by saying it was meant to replenish the fuel supply of a Tehran medical research reactor. But Western officials are sceptical, saying Iran had refined far more than it required for such a purpose.

While Iran stopped 20 percent enrichment in January, it is allowed under the November pact to keep producing uranium refined to up to 5 percent, the level required for fuelling civilian nuclear power stations.

Senior Iranian nuclear negotiator Abbas Araghchi was quoted by IRNA on Sunday as saying Iran would discuss all areas of their nuclear programme to come to a "quick" resolution.

He said the "drafting of the final nuclear deal" would begin in Vienna. Talks will start on Tuesday with a working dinner between EU foreign policy chief Catherine Ashton and Iranian foreign minister Mohammad Javad Zarif, according to IRNA.</s>Differences over how fast Iran could "break out" a nuclear weapon complicate the quest for a deal by late July under which Iran would scale back its atomic energy programme in exchange for a lifting of crippling sanctions.

The West wants Iran to significantly cut back its uranium enrichment capacity to ensure it would not be able to quickly produce a nuclear bomb. Iran says it needs to expand its programme to fuel a planned network of atomic energy plants, denying accusations of a secret nuclear weapons agenda.

With only some five weeks to go before the self-imposed July 20 deadline for a deal, the negotiating positions between Tehran and the United States, France, Germany, Russia, China and Britain remain far apart. A fifth round of talks since February is being held in Vienna this week.

U.S. Secretary of State John Kerry has said that Iran has the ability to produce highly-enriched uranium for one bomb in two months, if it so decided. Western officials and experts say this potential timeline must be substantially extended under any deal to end the decade-old nuclear dispute.

In an apparent attempt to counter that view, an Iranian website this month published a report saying it would take at least 18 months to do so, a time frame that would reach three years if conversion of the material into uranium metal and moulding - steps required to make a bomb - were included.

Yet more time would be needed to develop a vehicle, a missile for example, for a nuclear warhead to be delivered to its target.

"It is impossible for Iran to break out in months through the uranium route. The required time span is in years," the report published on www.nuclearenergy.ir said, stressing that this was a "hypothetical" scenario.

The website says it is "dedicated to providing accurate and factual information" about the Iranian nuclear programme, which the Islamic Republic says is entirely peaceful.

The Institute for Science and International Security (ISIS), a U.S.-based think-tank which closely monitors Iran's atomic activities, said on Wednesday that the Iranian website "expresses common government stances" on nuclear issues.

"This study contains mistakes and uses unwarranted assumptions to arrive at its conclusions," ISIS said of the Iranian report. "Using its data and correcting for mistakes, we arrive at a breakout estimate of 2-3 months in terms of the time to produce 25 kg of WGU (weapons-grade uranium)." That is the amount traditionally seen as sufficient for one bomb.

Uranium enriched to a fissile concentration of around 5 percent is used to fuel civilian nuclear power plants, Iran's stated ambition. If processed to a much higher degree, to about 90 percent, it can provide the fissile core of a nuclear bomb, which the West fears may be Tehran's covert aspiration.

Iranian Foreign Minister Mohammad Javad Zarif, the country's chief nuclear negotiator, criticised what he called the breakout "myth".

"For years, small but powerful constituencies have irrationally advanced the idea that Iran can produce enough fissile material for a bomb in months," he said in a June 13 article for the Washington Post.

If Iran ever wanted to take such a step, he said, it would have to expel U.N. nuclear inspectors and reconfigure its enrichment programme to make weapon-grade fissile material, which would then need to be turned into metal and "undergo countless other complex weaponisation processes".

"None of these capabilities exist in Iran and would have to be developed from scratch. This would take several years — not a few months," Zarif said.</s>Iran's foreign minister said a possible nuclear deal between Iran and six major powers would not be announced on Sunday.

"We still have got work to do and no deal will be announced on Sunday ... also there will be no extension after tomorrow," Mohammad Javad Zarif told reporters.

Iran and the powers said they were close to nailing down a deal that would bring sanctions relief in return for curbs on Tehran's nuclear program.</s>On Sunday, Iranian Supreme Leader Ayatollah Ali Khamenei described as "stupid and idiotic" Western expectations for his country to curb its missile program. He decreed mass production of ballistic weapons, striking a defiant tone just before nuclear talks resumed on Wednesday in Vienna.

The high-stakes negotiations aim for a deal by a July 20 deadline to end a long stand-off that has raised the risk of a wider Middle East war.

Tehran's often repeated view that missiles should not be part of the nuclear talks appears to enjoy the support of Russia, one of the six global powers.

But a senior U.S. official made clear this week that Tehran's ballistic capabilities must be addressed in the negotiations since U.N. Security Council resolutions on Iran "among many other things, do say that any missile capable of delivering a nuclear weapon must be dealt with."

A ban on developing missiles suited to carrying a nuclear warhead is included in a 2010 Security Council resolution, its fourth - and toughest - imposed on the Islamic Republic for defying council demands that it suspend uranium enrichment and other nuclear activities of potential use in bomb-making.

The new report by the U.N. Panel of Experts, seen by Reuters, said Iran's overall attempts to procure materials for its nuclear and missile programs appeared to have slowed down as it pursues negotiations with world powers that it hopes will bring an end to international sanctions.

But the same report makes clear that, apart from holding off on test-firing one type of rocket, Iran shows no sign of putting the brakes on the expansion of its missile program.

"Iran is continuing development of its ballistic missile and space programs," the experts said, citing the August 2013 identification of a new missile launch site near Shahrud and a larger missile and satellite launch complex at the Imam Khomeini Space Center at Semnan believed to be near completion.

The report also noted what it described as the June 2013 opening of the Imam Sadeq Observation and Monitoring Center for monitoring space objects, including satellites.

The dispute over missiles has already surfaced behind closed doors in Vienna. On Wednesday, the first day of the latest round of the nuclear talks, the U.S. delegation made clear that it wanted to discuss both Iran's ballistic missile program and possible military dimensions of its past nuclear research.

But in a sign of the wide divergence between the U.S. and Iranian positions, Iranian Foreign Minister Mohammad Javad Zarif merely laughed and ignored the remarks, according to an Iranian official present. An American official declined to comment but referred to remarks from a senior U.S. official earlier this week, who said "every issue" must be resolved.

Diplomats close to the talks say Britain, France and Germany agree with the U.S. view. But Russia, which has engaged in missile-technology trade with Iran, seems to disagree. Russian Foreign Minister Sergei Lavrov was quoted by Iranian media as saying that Tehran's missile program was not on the agenda.

The Islamic Republic denies accusations that it is seeking the capability to make nuclear weapons. It insists that its missiles are part of its conventional armed forces and rules out including them on the agenda for the nuclear discussions.

Speaking to reporters at a meeting with U.S. Defense Secretary Chuck Hagel, Israeli Prime Minister Benjamin Netanyahu raised the U.N. panel's report, saying it showed how Tehran works "to deceive the international community to continue to develop ICBMs (intercontinental ballistic missiles).

"As the talks continue, one thing that must guide the international community and that is we must not let the ayatollahs win, we mustn't let the foremost terrorist state of our time, Iran, develop the capability to produce nuclear weapons," the right-wing premier said in Jerusalem.

Hagel responded: "I want to assure you of the United States' commitment to ensuring Iran does not get a nuclear weapon - and that America will do what we must to live up to that commitment - which is what President Obama said here in Israel last year."

Netanyahu has long made clear he believes talking with Iran is the wrong approach and has clashed with Obama over it, and Israel has threatened to bomb Iranian nuclear installations if it deems diplomacy to have failed.

Netanyahu described an interim deal that the global powers struck with Iran in November as "an historic mistake".

However, Daryl Kimball of the Arms Control Association, a Washington-based research and advocacy group, said that missiles should not become a deal-breaker with the Iranians.

"The best way to address Iran's potential to exploit nuclear-capable missiles is to ensure that Iran's nuclear program is sufficiently limited and transparent," he said.

"To seek Iran-specific limits on conventional weapons that Iran regards as vital to its self-defense would jeopardize the negotiations' key objective."

An Iranian official confirmed that the ballistic missile program would not be interrupted. "Iran purchases parts from various countries, including Russia and China and then assembles missiles in Iran," he said.

"Some Gulf countries have been involved in the missile delivery to Iran. Iran has never stopped its missile program and has no intention to do so; it gives Iran an upper hand."

The U.N. Panel of Experts, which monitors compliance with the U.N. sanctions regime against Iran, said in its 49-page report that monitoring Iranian missile work was not easy.

"Analysis of Iran's ballistic missile program remains a challenge. With the exception of several launches, periodic displays of hardware and one recent revelation of a new ballistic launch facility, the program is opaque and not subject to the same level of transparency that Iran's nuclear activities are under IAEA safeguards."

It said procurement for the missile program continues, with no apparent changes in the type of materials Iran seeks.

"Among the most important items Iran is reportedly seeking are metals as well as components for guidance systems and fuel," the panel report said. "Similarities between Iran's ballistic missiles and space programs can make it difficult for states to distinguish the end-uses of procured items."

The experts said it was unclear why Iran appears not to have test-fired a Sejil, its longest-range, solid-fuelled ballistic missile, since 2011. This might be due to satisfaction with its performance, an inability to procure components or ingredients for solid fuel, or a shift to other missiles considered to be of higher priority, according to the report.

"Iran may also have decided to suspend further testing which could be interpreted as inconsistent with the spirit of the (six power) negotiations," the experts assessed.

Nevertheless, the panel said that proof Iran is continuing to develop the Sejil came from a 2013 parade of their launchers.

On February 10, Iran test-fired the Barani, which the experts said the Iranian Defense Ministry had described as "a new generation of long-range ballistic missiles capable of carrying multiple re-entry vehicle (MRV) payloads." A MRV payload deploys multiple warheads in a pattern against a single target.

Iran had announced no other ballistic test, the panel said.

According to Jane's Defense Weekly, Iran recently unveiled an indigenous copy of the Lockheed Martin RQ-170 unmanned aerial vehicle as well as "other, potentially more significant, revelations" - including new versions of the Fateh-110 tactical ballistic missile known as the Hormuz-1 and Hormuz-2.

Analysts say, however, that Iran has tended to exaggerate its military achievements, including its missile capabilities.</s>After more than two weeks of marathon negotiations, Iranian and Western officials said the earliest an agreement could be ready was more likely Monday.

"We are working hard, but a deal tonight is simply logistically impossible," Alireza Miryousefi, a spokesman for the Iranian delegation, said on Twitter. "This is a 100-page document, after all."

In a sign that prospects for a deal on Sunday were vanishing, British Foreign Secretary Philip Hammond was on his way back to London, though he was expected to return on Monday, officials said.

French Foreign Minister Laurent Fabius cancelled a planned trip to Central African Republic and Ethiopia due to the continued deadlock in the Iran negotiations, a French diplomatic source said.

U.S. Secretary of State John Kerry cautioned that some difficult issues remained on the 16th day of ministerial negotiations between Iran, the United States, Britain, France, Germany, Russia and China.

Several diplomats had said an agreement that would end more than a year and a half of negotiations was so close that it could come as early as Sunday. In a sign that something might be in the works, Russian Foreign Minister Sergei Lavrov arrived in Vienna and Chinese Foreign Minister Wang Yi was due on Sunday, but the schedule for ministerial talks was unclear.

However, a senior U.S. official played down speculation that an agreement was in the works on Sunday, and reiterated Kerry's point that "major issues remain to be resolved in these talks."

Fabius said he hoped the high-stakes negotiations were finally drawing to a close.

"I hope, I hope, that we are finally entering the final phase of this marathon negotiation," Fabius told reporters.

A senior Iranian official also said an agreement was close.

"Some 99 percent of the issues have been resolved and the agreement is ready," said an Iranian diplomat. "With political will, we can finish the work late tonight and announce it tomorrow. But still there are at least two issues to be resolved."

Iran and the six powers involved in the talks have given themselves until Monday to reach a deal, their third extension in two weeks, as the Iranian delegation accused the West of throwing up new stumbling blocks to an accord.

Among the biggest sticking points this week has been Iran's insistence that a United Nations Security Council arms embargo and ban on its ballistic missile programme dating from 2006 be lifted immediately if an agreement is reached.

Russia, which sells weapons to Iran, has publicly supported Tehran on the issue.

Western powers have long suspected Iran of aiming to build nuclear bombs and using its civilian atomic energy programme to cloak its intention - an accusation Iran strongly denies.

The goal of the deal is to increase the time it would take for Iran to produce enough enriched uranium fuel for a single weapon to at least one year from current estimates of 2-3 months - the "breakout" time.

If there is a deal, the limits on Iran's enrichment programme are expected to be in place for at least a decade.

Other problematic issues in the talks are access for inspectors to military sites in Iran, answers from Tehran over past activity and the overall speed of sanctions relief.

Kerry and Zarif have met nearly every day since Kerry arrived in Vienna more than two weeks ago for what was intended to be the final phase in a negotiation process that began with an interim nuclear deal clinched in November 2013.

Kerry was meeting with Zarif on Sunday, a U.S. official said, in an attempt to break the deadlock.

Experts and senior officials from Iran, the United States and the other powers have been meeting non-stop for months, often working into the early hours of the morning, to finalise an accord that will include five technical annexes.

An agreement would be the biggest step towards rapprochement between Iran and the West since the 1979 Islamic Revolution, although both sides are likely to remain wary of each other even if a deal is concluded.

Iranian Supreme Leader Ali Khamenei said Tehran would continue its fight against "global arrogance" - referring to the United States. But Khamenei did not set any new "red lines" for his negotiators as he did in a tough speech two weeks ago.

In Washington, the top Republican in the U.S. Senate cast doubt on whether President Barack Obama will be able to win approval in Congress for any deal.

"I think it's going to be a very hard sell, if it's completed, in Congress," Senate Majority Leader Mitch McConnell said on the "Fox News Sunday" broadcast. "We already know it's going to leave Iran as a threshold nuclear state."</s>U.S. Secretary of State John Kerry was due to meet Iranian Foreign Minister Mohammad Javad Zarif with European Union envoy Catherine Ashton to try to break the stalemate, but talk of an extension prompted calls for tougher sanctions in Washington.

The United States and its allies had hoped this week's talks in Vienna would be the culmination of months of difficult diplomacy between Iran and the United States, Britain, France, Germany, Russia and China on a comprehensive agreement.

The aim is to remove sanctions on Tehran in exchange for curbs on its atomic programme. But the talks have long been deadlocked. The timing for lifting sanctions and future scope of Iran's uranium enrichment are stumbling blocks.

"Important points of difference remain," French Foreign Minister Laurent Fabius told a news conference in Paris with Kerry.

The U.N. nuclear chief, Yukiya Amano, on Thursday highlighted another hurdle: Iran has yet to explain suspected atomic bomb research to the U.N. nuclear agency, one of the six powers' conditions for lifting sanctions.

The latest round of talks between the six and Iran began on Tuesday and are likely to last right up to a self-imposed Nov. 24 deadline for a final agreement.

"Some kind of interim agreement at this point is likely, or perhaps at best a framework agreement by Monday that needs to be worked out in the coming weeks and months," a Western diplomat told Reuters on condition of anonymity.

"We need more time to resolve technical issues and don’t forget that the time frame for lifting sanctions is still a huge dispute," the Iranian official said, adding that an extension until March was a possibility. Western officials also suggested March was an option, with a resumption of talks in January.

The officials said, however, that Iran and the six were not actively discussing an extension yet and would push for a deal by the deadline, which has already been extended from July.

"The talks have been going in an overwhelmingly tense atmosphere. Time is running out," Russian Deputy Foreign Minister Sergei Ryabkov was quoted as saying by RIA Novosti news agency. Various meetings were under way and no-one was talking about prolonging the talks, he said.

Some hawkish members of U.S. Congress, citing Iran's nuclear programme as a threat to Israel, expressed frustration over the prospect of an extended deadline. Many Republicans, who will control both houses of Congress from January, insist stiffer sanctions will force Iran to compromise.

"The thing that the Iranians need most is time to complete their atomic bomb. The president shouldn't give it to them," said Republican Senator Mark Kirk, co-author of a bill to slap tough sanctions on Iran.

Officials close to the negotiations, which began in February, say Iran wants all key sanctions on oil exports and banking terminated almost immediately, not merely suspended as the United States and European officials have said.

Tehran rejects Western allegations it is amassing the capability to produce atomic weapons and has refused to halt its enrichment programme.

It has been under international sanctions for eight years and the U.S., European Union and U.N. measures have slashed its oil exports, causing inflation to soar and the value of its currency to plummet.

Western powers say the sanctions can be suspended gradually and in line with moves by Iran to restrain its nuclear activities and terminated only after Tehran has demonstrated full compliance with the terms of any deal.

They also want Iranian restrictions to last for 20 years, while Iranians are pushing for either months or a small number of years, diplomats say.

Western officials say Iran has refused to budge on enrichment, despite repeated offers of potential compromises.

Tehran may consider keeping fewer enrichment centrifuges as long as they are advanced, to keep the volume unchanged, Western officials say, adding that this represents no compromise at all.

They Iran is unwilling to compromise largely because Supreme Leader Ayatollah Ali Khamenei has not given the negotiators the freedom to do so.

"The ball is in the Iranian camp and to be honest we have a feeling that we're treading water at the moment," a senior Western diplomat said. "The main obstacle is that the decisions have to be made by the Iranian leadership."

The Iranians pin the blame on Western powers they accuse of expecting too much.

Iranian Foreign Minister Mohammad Javad Zarif said Iran would resist Western pressure to make what it considered to excessive concessions in the Vienna talks.</s>Iran and world powers are engaged in talks to reach a comprehensive deal that would place limits on Iran's nuclear program in return for the removal of some U.S., European, and United Nations sanctions on its financial and energy sectors.

The United States has said that sanctions removal on Iran will be phased gradually, but Iranian officials, including Supreme Leader Ali Khamenei, have said that sanctions on Iran must be lifted as soon as a final agreement is concluded.

"Solutions have been obtained and it seems that the principles and red lines of the Islamic Republic in technical issues have been accepted by the enemy," said Mohammad Ali Jafari, the commander of Iran's Islamic Revolutionary Guard Corps (IRGC), according to Fars News.

"However in regards to removing the sanctions, there are ambiguities which need to be made clear and we must realize that this very issue of how the sanctions will be removed can lead to a lack of agreement."

The IRGC, an elite military force, plays a major role in Iran's political and economic affairs.
Tasnim News Agency reports that, in a Nowruz speech, Supreme Leader of Iran Ali Khamenei said experts in his country can enrich uranium to 99 percent. ISNA reports that Iranian Foreign Minister Mohammad Javad Zarif is threatening to quit the Iran nuclear deal if the United States blocks Iran's version of the deal's implementation.