LAUSANNE, Switzerland — With 10 days to a nuclear deal deadline, top U.S and Iranian officials spoke Saturday of substantial headway, and Iran's president proclaimed that agreement was within reach. But America's top diplomat said it was up to Tehran to make the decisions needed to get there.

Iranian President Hassan Rohani said "achieving a deal is possible" by a March 31 target date for a preliminary accord that is meant to lead to a final deal by the end of June that would crimp Tehran's nuclear programs in exchange for sanctions relief.

U.S. 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," 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.

Kerry was departing later Saturday to meet with European allies in London, in part to ensure unity, before returning to Washington. Kerry said the U.S. 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."

On Twitter on Friday, France's ambassador to the U.S. called talk about needing a deal by March 31 a "bad tactic" that is "counterproductive and dangerous." Gerard Araud called it an "artificial deadline" and said negotiators should focus instead on the next phase — reaching a complete agreement by the end of June.

Kerry said the U.S. wasn't rushing into a pact, stressing that the latest stab at a diplomatic settlement with Iran has gone on for 2 ½ 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," 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."

One encouraging sign is the apparent narrowing of differences on Iran's uranium enrichment program. Tehran insists it wants to enrich only for energy, medical and research purposes, but much of the world fears it could turn the process toward making the fissile core of a nuclear warhead.

As the current round wound down this week, officials told The Associated Press that the United States and Iran are drafting elements of a deal that commits the Iranians to a 40 percent cut in the number of machines they use to enrich. The Obama administration is seeking a deal that stretches the time Tehran would need to make a nuclear weapon from the present two to three months to at least a year.

For Washington, the stakes are high if the talks miss the March deadline. The Obama administration has warned that a diplomatic failure could lead to an ever tougher dilemma: Whether to launch a military attack on Iran or allow it to reach nuclear weapons capacity.

A more immediate challenge may be intervention from Congress. If American lawmakers pass new economic sanctions on Iran, the Islamic Republic could respond by busting through the interim limits on its nuclear program it agreed to 16 months ago. Thus far, it has stuck to that agreement.