French Foreign Minister Jean-Yves Le Drian hosted his German and British counterparts in Paris, with America's new Secretary of State Antony Blinken joining via videoconference.

Analysts say only a small window of opportunity remains to save the deal, which received a near-fatal blow when former US president Donald Trump walked out of the accord in 2018 and reimposed sanctions on Iran.

Tehran retaliated by stepping up nuclear work in violation of the accord.

The three European ministers were scheduled to meet from 1500 GMT, with Blinken joining by video conference from Washington at 1630 GMT.

"The recent steps of Iran are not helpful at all, they endanger the return of the Americans" to the deal, German Foreign Minister Heiko Maas told reporters in Paris.

"Apparently Iran is not interested in easing the tensions, but in escalation. They are playing with fire," he said.

The administration of US President Joe Biden has said it is prepared to rejoin the deal and start lifting sanctions if Iran -- whose economy has been devastated -- returns to full compliance.

But Tehran rejected this precondition, pressing on with increasing nuclear work in retaliation for Trump's so-called "maximum pressure" sanctions policy to weaken the Iranian regime which has had no relations with Washington for four decades.

The Joint Comprehensive Plan of Action (JCPOA), signed in Vienna in 2015, was based on Iran providing safeguards that it would not make an atomic bomb, in exchange for a gradual easing of international sanctions.

- 'Serious impact' -

The diplomacy is expected to be hugely delicate and could be further derailed by the deadline set under a bill adopted by the Iranian parliament in December following the killing of top nuclear scientist Mohsen Fakhrizadeh, which Tehran blamed on Israel.

Iran would restrict some UN nuclear agency inspections by February 21 if the US does not lift the sanctions imposed since 2018.

International Atomic Energy Agency (IAEA) chief Rafael Grossi is to travel to Tehran on Saturday for talks with the Iranian authorities to find a solution for continuing inspections in the country, the agency said.

It warned that the step threatened by Tehran would have "a serious impact on the IAEA's verification and monitoring activities in the country".

In Washington, State Department spokesman Ned Price said that Iran should provide "full and timely cooperation" with the IAEA and reverse its recent steps violating the accord.

The IAEA said last week that Iran had started producing uranium metal in a new violation of the accord, intensifying concerns it was becoming closer to having the capacity to make a nuclear weapon.

"Iran's nuclear program is growing by the day, as the time it would take to enrich enough uranium for a single nuclear weapon shrinks," said Ali Vaez, Iran project director at the International Crisis Group (ICG).

- 'Solution for dilemma' -

While Iran's policy is ultimately determined by supreme leader Ayatollah Ali Khamenei, Iranian presidential elections in June add another time pressure factor.

Rouhani -- a key advocate of nuclear diplomacy with global powers -- is set to step down after serving the maximum two consecutive terms, and a more hardline figure is likely to replace him.

Ellie Geranmayeh, senior policy fellow at the European Council on Foreign Relations, said Washington should move in political and practical terms to show Iran that the Biden administration "is distancing itself from Trump-era maximum pressure."

"There is a short window of time to limit the damage that could ensue from Iran's next steps, for example by reducing the impact of such moves on the quality of inspections by international monitors," Geranmayeh told AFP.

Vaez said "the seemingly impossible dilemma has a solution" if the two sides were prepared to take "closely synchronised steps".

This would involve Washington revoking Trump's 2018 withdrawal and greenlighting an Iranian request for an emergency IMF loan, while Iran freezes "the most problematic aspects" of its nuclear programme, he said.

Khamenei emphasised Wednesday that Iran wanted to see "only action, action" from the United States.

"If we see action from the opposite side, we will act too," he said.