Iran's president has proclaimed that a nuclear agreement is in reach, as a deadline looms for a preliminary accord.
Iranian President Hassan Rohani said "achieving a deal is possible" by the March 31 target date that is meant to lead to a final deal by the end of June that would scale back Tehran's nuclear programmes in exchange for sanctions relief.
However US Secretary of State John Kerry was more circumspect, as he spoke to reporters after six days of negotiations in the Swiss city of Lausanne. The talks, made "substantial progress," he said, but "important gaps remain".
"We have an opportunity to get this right," Mr Kerry said, as he urged Iran to make "fundamental decisions" that prove to the world it has no interest in atomic weapons.
But Iran's supreme leader warned against expectations that even a done deal would mend the more than three-decade freeze between the two nations in place since the Iranian revolution and siege of the American Embassy, proclaiming that Washington and Tehran remained on opposite sides on most issues.
"Negotiations with America are solely on the nuclear issue and nothing else. Everyone has to know that," Ayatollah Ali Khamenei told a crowd in northeastern Iran on the first day of the Persian new year. "We do not talk with US over regional issues. In the regional issues, America's goals are completely opposed to our goals."
In a reflection of the delicate state of negotiations, other officials differed on how close the sides were to a deal.
Top Russian negotiator Sergey Ryabkov and Iran's atomic energy chief Ali Akbar Salehi said in recent days that technical work was nearly done. But French officials insisted the sides were far from any agreement.
Mr Kerry was departing later today to meet with European allies in London, in part to ensure unity, before returning to Washington. Mr Kerry said the US and its five negotiating partners - Britain, China, France, Germany and Russia - are "united in our goal, our approach, our resolve and our determination".
But France, which raised last minute objections to an interim agreement reached with Iran in 2013, could threaten a deal again. It is particularly opposed to providing Iran with quick relief from international sanctions and wants a longer timeframe for restrictions on Iran's nuclear activity.
"France wants an agreement, but a robust agreement," French Foreign Minister Laurent Fabius told French radio. "That is to say, an accord that really guarantees that Iran can obviously have access to the civil nuclear (program)."
"But to the atomic bomb? No."
Mr Kerry said the US wasn't rushing into a pact, stressing that the latest stab at a diplomatic settlement with Iran has gone on for two and a half years.
"We don't want just any deal," he said. "If we had, we could have announced something a long time ago."
But, he added, decisions "don't get any easier as time goes by".
"It's time to make hard decisions," Mr Kerry said. "We want the right deal that would make the world, including the United States and our closest allies and partners, safer and more secure. And that is our test."