After years of distrust, is Iran's nuclear deal near ? NEW: German Foreign Ministers highlights significant differences that need working out John Kerry and other top diplomats flock to Geneva as talk of a deal buzzes Spokeswoman: Kerry going there "with the hope that an agreement will be reached" Iran's foreign minister says they are 90% of the way there (CNN) -- After years of disagreement and distrust, a deal between Iran and international powers over its nuclear program seemed near Saturday as top diplomats hastily flocked to the site of ongoing talks. But mixed in with the signs of hope, were also some sobering words. U.S. Secretary of State John Kerry landed in Geneva in the morning and went straight to the venue where talks are being held. He "made the decision to travel here with the hope that an agreement will be reached," State Department spokeswoman Marie Harf had said. A day before, a Western official said that a deal was within grasp. And at the beginning of Saturday's working meeting, the mood was still positive, a leading European Union diplomat told CNN on condition of anonymity. The Iranians seem to share the sentiment. "Not bad," was the assessment of progress by an Iranian diplomat, who did not wish to be named. Curb the enthusiasm But two European foreign policy chiefs made a point of curbing the enthusiasm. "It's not a done deal," said Germany's Foreign Minister Guido Westerwelle, who walked out of the negotiation venue to give the statement to journalists. Significant differences still stand in the way, and the world's most powerful diplomats have come to do their best to work them out. "We think there is a realistic chance, but there is still a lot of work to do," he said, before turning away from the microphones. Britain's Foreign Secretary William Hague backed up his colleague's caution. We're not here because things are necessarily finished. We're here because they're difficult. Narrow gaps remain, but they are important ones. P5+1 rushes in The EU's foreign policy chief, Catherine Ashton continues to lead the talks with Iranian Foreign Minister Javad Zarif and his counterparts from other countries, the EU diplomat said. Zarif said Friday there is wide agreement except for a couple of points, the semiofficial Iranian Students News Agency reported. "Numerically speaking, perhaps 90% of progress has been made, but there (are) one or two issues which are of great significance," he said. Kerry joins Westerwelle, Hague, French Foreign Minister Laurent Fabius, Chinese Foreign Minister Wang Yi and their Russian counterpart, Sergey Lavrov. Together, these diplomats represent the five permanent members of the U.N. Security Council plus Germany -- together known as the P5+1 -- which has been negotiating with Iran on its nuclear program. The snags Western powers want Iran to restrain technical advancements that could put it close to developing a nuclear weapon. Tehran wants economic sanctions that are strangling its economy loosened. But Iran also insists it has the right to enrich uranium, saying that it will only be used for a peaceful nuclear energy program. Tehran wants that written into the deal, which would likely cover a period of six months and ideally be a precursor to a more sweeping pact, diplomats said. Western powers, on the other hand, prefer ambiguity on this matter. They don't want it written into the agreement. But if Iran states it has the right to enrich uranium, the West won't argue with it, the diplomats said. Washington and its allies also only want to lift only some of the sanctions and leave some tough ones in place for now. Change in tone For years, Iran and Western powers have left negotiating tables in disagreement, frustration and at times open animosity. But the diplomatic tone changed with the transfer of power after Iran's election this year, which saw President Hassan Rouhani replace Mahmoud Ahmadinejad. Caustic jabs at the United States and bellicose threats toward Israel were a hallmark of Ahmadinejad's foreign policy rhetoric. He lambasted the West over the economic sanctions crippling Iran's economy and at the same time, pushed the advancement of nuclear technology in Iran. Rouhani has struck up a more conciliatory tone and made the lifting sanctions against his country a priority. Despite the sanctions, Iran today has 19,000 centrifuges and is building more advanced ones, according to Mark Hibbs, a nuclear policy expert at the Carnegie Endowment for International Peace. Most world powers believe that Iran could not realistically build a usable bomb in less than a year, Hibbs said. And Iran recently signed a deal with the International Atomic Energy Agency that agrees to give the U.N. nuclear watchdog agency access to long-unseen nuclear sites, including a heavy-water reactor in Arak. Tehran is also a party to the Treaty on the Non-Proliferation of Nuclear Weapons, which requires it not to create nuclear weapons or enable other countries to obtain them. Lingering distrust Key U.S. allies in the Middle East and some Washington lawmakers still don't trust Iran. They believe that by making concessions on sanctions, the United States and its allies are giving up important leverage against Tehran. Saudi Arabia, Iran's nearby neighbor across the Persian Gulf, has lasting tensions with Tehran and has publicly derided the Obama administration's negotiating stance. A bipartisan group of six senators urged the President's team to reject the proposed deal and accept only an agreement that better dismantles Iran's nuclear technology. Israeli leaders, as well, would like to see the heat turned up on Tehran, not down. Defense Minister Moshe Ya'alon reiterated the point after a meeting with U.S. Secretary of Defense Chuck Hagel on Saturday in Israel. We think the deal that is on the table is a bad one and even if this deal is signed there will be a lot to do afterward to bring the Iranian regime face to face with the dilemma of the bomb or survival. Israel has said that it reserves the right to defend itself militarily against an Iranian nuclear threat. Israeli Prime Minister Benjamin Netanyahu has been adamant in his distrust of Tehran and his belief that sanctions are working and should get tougher. But President Barack Obama said that sanctions put in place during his administration had forced Iran to the negotiating table and easing them some could help move things forward. The proposed deal would only "open up the spigot a little bit" on frozen revenue, while leaving in place the bulk of the most effective sanctions involving Iranian oil exports and banking. The President also stressed that all options, including military strikes on Iran's nuclear facilities, remained on the table as far as the United States was concerned. CNN's Shirley Henry, Nick Thompson, Alex Felton, Tom Cohen and Greg Botelho contributed to this report.