US Secretary of State Antony Blinken expressed his fears that it may be too late to restore the 2015 nuclear deal with Iran if talks drag on.
"We still have significant differences with Iran," Mr Blinken told an audience in Paris on Friday.
The Vienna discussions on readmitting the US to the Joint Comprehensive Plan of Action (JCPOA) that set some limits on Iran's atomic programme failed to meet expectations of a deal in the latest round, which broke up last week.
"There will come a point, yes, where it will be very hard to return to the standards set by the JCPOA," he said.
If Iran "continues to spin ever more sophisticated centrifuges" and steps up uranium enrichment, Mr Blinken said, the "breakout" time – the point at which Tehran will be dangerously close to the ability to develop a nuclear bomb – would undermine the deal's limits.
Mr Blinken said that US President Joe Biden continues to support a return to the accord, under which Iran had scaled back its nuclear work, until former president Donald Trump withdrew in 2018 and imposed sanctions on Tehran.
"We have a national interest in trying to put the nuclear problem back in the box that it was in the JCPOA," Mr Blinken said.
"We haven't reached that point – I can't put a date on it – but it's something that we're conscious of."
Mr Blinken also met the French president Emmanuel Macron and a statement said the two men discussed security challenges as well as the crisis in Lebanon. "Secretary Blinken and President Macron exchanged views on countering terrorist threats, supporting democracy, and joint efforts to improve the capacity of our African partners in the Sahel region," the State Department said. "They also discussed efforts underway with African and other partners to address the humanitarian and human rights crises in Tigray, while emphasising the need for Lebanon’s leaders to come together for the good of the Lebanese people."
France on Friday urged the Iranian government to make the final decisions to allow the revival of the 2015 nuclear deal through the re-entry of the US to the accord.
"We expect the Iranian authorities to make the final decisions – no doubt difficult ones – which will allow the negotiations to be concluded" in Vienna, French Foreign Minister Jean-Yves Le Drian said.
Meanwhile, the diplomats were also concerned about the lapse of an interim monitoring deal the International Atomic Energy Agency said expired on Thursday.
It is in talks with Iran for another extension.
"This remains a serious concern," Mr Blinken said. "The concern has been communicated to Iran and needs to be resolved."
Mr Blinken, who was raised in Paris, saluted the alliance with France and sprinkled his remarks with fluent French, in a sharp change of tone after the sometimes abrasive "America First" approach of the Trump administration.
"My dear Tony, I'm really very happy to welcome you to Paris," Mr Le Drian said, as he received Mr Blinken in an ornate room of the Quai d'Orsay, the French Foreign Ministry.
"It's expected that you would visit Paris because you're at home here. I would even be tempted to say, welcome home!"
Mr Blinken is on a European tour that has taken him to Germany and will continue in Italy, after a recent visit by Mr Biden to the continent.
The administration is looking to solidify relations with European governments in the face of growing challenges from China and Russia.
On hotspots of strategic importance to the French, Mr Blinken promised solidarity on tackling extremism in the Sahel and a united front on Lebanon.
"We have decided to act together to put pressure on those responsible. We know who they are," Mr Le Drian said of Lebanon, which is engulfed in economic and political crises.
"We need to see real leadership in Beirut," Mr Blinken said.
He is scheduled to meet Pope Francis and Archbishop Paul Richard Gallagher, the Pope's de facto foreign minister, on Monday at the Vatican.
Pope Francis has supported the fight against climate change, called for greater compassion towards refugees and played a key role in behind-the-scenes diplomacy with the administration of Barack Obama that worked towards normalising relations with Cuba – a process reversed by Mr Trump.
The US secretary of state said that the first priority of the US in the context of Israel and Palestine was assisting the people of Gaza after the recent conflict.
