French President Emmanuel Macron's top diplomatic adviser on Wednesday held talks in Tehran aimed at saving the 2015 nuclear deal and easing tension between Iran and the US.
The meetings came as a US official described Tehran's breaches of the agreement, from which America withdrew last year, as "nuclear extortion".
Emmanuel Bonne met Rear Admiral Ali Shamkhani, the secretary of Iran's Supreme National Security Council, and Foreign Minister Javad Zarif and his deputy Abbas Araghchi.
Mr Bonne's mission is "to try to open the discussion space to avoid an uncontrolled escalation, or even an accident", said Jean-Yves Le Drian, the French Foreign Minister.
Iran has dropped some commitments to the nuclear deal with world powers in response to heavy US sanctions and what it calls a lack of success by European countries to find ways around them.
The US urged world powers that were party to the nuclear deal to impose sanctions on Iran and pressure it into reversing its recent increase in uranium stockpiles and threats of enriching levels beyond the limits in the deal.
Jackie Wolcott, the US ambassador to international organisations in Vienna, told the International Atomic Energy Agency on Wednesday that Iran's recent "misbehaviour" should not be rewarded.
Ms Wolcott said Washington believed the deal was ineffective but was "open to negotiation without preconditions".
But Tehran said it would not negotiate with the US until the sanctions were dropped.
"Negotiations are never possible under pressure," Mr Zarif said before the meeting with Mr Bonne.
But Foreign Ministry spokesman Abbas Mousavi "welcomed France's role in reducing tension and implementing the accord", state news agency Irna reported.
Iran threatened on Monday to restart deactivated centrifuges and increase enrichment of uranium to 20 per cent purity in a move away from the nuclear accord.
Its moves have been rebuked in capitals across Europe. The three European signatories to the deal, Britain, France and Germany, have called on Iran to reverse its actions.
"It must act accordingly by reversing these activities and returning to full compliance without delay," said a statement from the EU and foreign ministers of the three countries on Tuesday.
Iranian President Hassan Rouhani said on Wednesday that Iran's decision to increase enrichment would produce fuel for power plants and serve other peaceful aims, and that it was within the framework of Iran's 2015 nuclear deal with world powers.
"You do not need to worry about Iran," Mr Rouhani told the EU and foreign ministers.
"Your concern must be over the United States, which has violated this whole commitment and undermined international obligations."
The International Atomic Energy Agency said that while Iran consistently lived up to its commitments under the deal until recently, it is now in breach of two of them.
One of the ideas mooted by Paris is that there should be a "freeze against freeze" plan, by which Iran would slow its breaches of the nuclear accord.
Mr Mousavi would not say whether the French envoy's visit was organised with Washington's knowledge.
Iran says it will continue to breach limits of the deal one by one until it receives the economic windfall the deal promised.
"It was a huge mistake by the Americans to leave the deal," Behrouz Kamalvandi, spokesman for Iran's Atomic Energy Organisation, said on Tuesday. "That has caused all the problems."