U.S. intelligence agencies have concluded in recent months that Iran has created enough nuclear fuel to make a rapid, if risky, sprint for a nuclear weapon. But new intelligence reports delivered to the White House say that the country has deliberately stopped short of the critical last steps to make a bomb. In the first public acknowledgment of the intelligence findings, the U.S. ambassador to the International Atomic Energy Agency declared on Wednesday that Iran now had what he called a 'possible breakout capacity' if it decided to enrich its stockpile of uranium, converting it to bomb-grade material. The statement by the ambassador, Glyn Davies, was intended to put pressure on U.S. allies to move toward far more severe sanctions against Iran this month, perhaps including a cutoff of gasoline to the country, if it fails to take up President Barack Obama's invitation for serious negotiations. But it could also complicate the administration's efforts to persuade an increasingly impatient Israeli government to give diplomacy more time to work, and hold off from a military strike against Iran's facilities. In interviews over the past two months, intelligence and military officials, and members of the Obama administration, have said they are convinced that Iran has made significant progress on uranium enrichment, especially over the past year. Iran has maintained that its continuing enrichment program is for peaceful purposes, that the uranium is solely for electric power and that its scientists have never researched weapons design. But in a 2007 announcement, the United States said that it had found evidence that Iran had worked on designs for making a warhead, though it determined that the project was halted in late 2003. The new intelligence information collected by the Obama administration finds no convincing evidence that the design work has resumed. It is unclear how many months -- or even years -- it would take Iran to complete that final design work, and then build a warhead that could fit atop its long-range missiles. That question has been the subject of a series of sharp, behind-the-scenes exchanges between the Israelis and top U.S. intelligence and military officials, dating back nearly two years and increasing in intensity in recent months. The American position is that the United States and its allies would probably have considerable warning time if Iran moved to convert its growing stockpile of low-enriched nuclear fuel to make it usable for weapons. While there is little doubt inside the U.S. government that Iran's ultimate goal is to create a weapons capability, there is some skepticism about whether an Iranian government that is distracted by the fallout from a disputed presidential election would take that risky step, and how quickly it could overcome the remaining technological hurdles. But Israel draws more dire pictures from the same set of facts. In classified exchanges with the United States, it has cited evidence that the design effort secretly resumed in 2005, at the order of Iran's supreme leader, Ayatollah Ali Khamenei. U.S. officials say that the evidence is circumstantial, and point out that the Israelis have not produced a copy of the order they say Khamenei gave. 'We're all looking at the same set of facts,' said one senior Israeli intelligence official, who, like others interviewed for this article, asked for anonymity because of the sensitive nature of the intelligence-gathering. 'We are interpreting them quite differently than the White House does.' At the core of the dispute is the 'breakout capacity' that Davies referred to on Wednesday in his first presentation as ambassador to the International Atomic Energy Agency, the United Nations' nuclear watchdog. The phrase refers to a non-nuclear nation's ability to acquire enough fuel and expertise to be able to complete building an actual weapon relatively quickly. The Israelis have contended that there could be little or no warning time -- especially if Iran has hidden facilities -- and they contended that in the aftermath of Iraq, the U.S. intelligence agencies are being far too cautious in assessing Iran's capability. As U.S. and Israeli officials expected, Iran turned over to European nations on Wednesday what it called a new set of 'proposals' for negotiations over its nuclear program. U.S. officials said they had not read them, but Susan E. Rice, the U.S. ambassador to the United Nations, said the Iranian response must be 'serious, substantive and constructive' to meet Obama's test. The White House has given Iran a late-September deadline to begin substantive negotiations, or face additional sanctions. Administration officials are debating whether the Iranian leadership, struggling with violent protests, is effectively paralyzed when it comes to negotiating with the West -- or for that matter in determining how aggressively to push ahead with its nuclear program. The White House is hoping its offer to negotiate has thrown Iran's leadership off track, and built up credibility around the world if the president begins to press for tougher sanctions. The intelligence updates for Obama follow the broad outlines of the conclusions delivered to President George W. Bush in 2007, as part of a 140-page National Intelligence Estimate. It was based on information gathered by U.S. spy agencies that had pierced Iran's military computer networks, coming up with surprising evidence that the country had halted its weapons-design effort four years earlier. Critics said the public portion of the report underemphasized the importance of Iran's progress in the enrichment of uranium, the hardest part of the bomb-making process. Accurate intelligence about the progress of Iran's weapons programs has been notoriously poor. Much of the country's early activity was missed for nearly 18 years, until a dissident group revealed the existence of enrichment efforts. Both the 2007 National Intelligence Estimate and the recent updates for Obama, according to officials familiar with their contents, are filled with caveats that Iran could be conducting uranium enrichment or weapons design work at remote locations that have eluded detection. The 2007 estimate outraged Israel, so much so that the next year the Israeli government secretly went to Bush to seek bunker-busting bombs, refueling capability and overflight rights over Iraq, in case it moved to strike Iran's facilities. He turned Israel down. Last month, former Vice President Dick Cheney told Fox News that he 'was probably a bigger advocate of military action than any of my colleagues.' In recent interviews, former Bush administration officials confirmed that they had asked the Pentagon to draw up possible attack scenarios. But the issue was never seriously debated because Defense Secretary Robert M. Gates and Secretary of State Condoleezza Rice were firmly opposed, the officials said, partly because they felt that an attack would not deal a significant setback to Iran's program. 'The vice president believed, and the Israelis believed, that it would be better if the Bush administration took care of it,' one former official said. By the last count of the international inspectors, Iran has installed more than 8,000 centrifuges -- the machines that enrich uranium -- at its main underground facility at Natanz, the primary target the Israelis had in their sights. At last inspection, Iran was using only a little more than half of them to enrich uranium. If Tehran has no hidden fuel-production facilities, creating a bomb would require it to convert its existing stockpile of low-enriched uranium into bomb-grade material. International inspectors, who visit Natanz regularly, would presumably raise alarms. Iran would also have to produce or buy a working weapons design, complete with triggering devices, and make it small enough to fit in one of its missiles. The official U.S. estimate is that Iran could produce a nuclear weapon sometime between 2010 and 2015, probably later rather than sooner. Israel's official estimate is close: Meir Dagan, the director of the Mossad, Israel's main spy agency, told the Israeli Parliament in June that unless action was taken, Iran would have its first bomb by 2014, according to an account in the Israeli newspaper Haaretz that Israeli officials have confirmed. 'Israel expects that the international community will prevent Iran from gaining nuclear military capabilities,' said Michael Oren, Israel's new ambassador to Washington. Despite Dagan's public comments, most Israeli officials believe that Iran could create a bomb much more quickly. They cite the murky evidence surrounding two secret programs in Iran, called Project 110 and Project 111. Those are the code names for what are believed to be warhead-design programs run by an academic named Mohsen Fakrizadeh. Iran has never allowed Fakrizadeh to be interviewed. But international inspectors have shown videos and documents suggesting that Fakrizadeh's group has worked on nuclear triggers, flight trajectories for missiles and the detonation of a warhead at almost 2,000 feet above ground -- which would suggest a nuclear detonation. On Wednesday, Iran repeated its assertion that this evidence consisted of 'forgeries' and 'fabrications.' Israeli officials say privately that the Obama administration is deluding itself in thinking that diplomacy will persuade Iran to give up its nuclear program. The Obama administration says it believes that Iran is on the defensive -- fearful of more crippling sanctions and beset by internal turmoil. But even inside the White House, some officials think Obama's diplomatic effort will prove fruitless. Some administration officials insist Israel is throwing out worst-case scenarios to 'shorten the timeline' to an Iranian bomb as a way to put pressure on the Obama administration. But some administration officials acknowledge that Israel's impatience and hints of military action are useful because they might push the Iranian government into negotiations, with real deadlines. At a meeting with a senior Obama administration official several months ago, Israeli officials pressed for intelligence and other help that would be necessary for a strike, according to one official with knowledge of the exchange. 