France said on Tuesday it was coordinating a response with world powers, including the United States, after Iran said it would begin enriching uranium at 60 percent.
The French presidency said the Iranian move was a "serious" development and needed coordination with the remaining parties to a 2015 nuclear deal, and Washington.
Iran said on Tuesday it will start enriching uranium to 60 percent purity, a move bringing the fissile material closer to levels suitable for a bomb, after accusing Israel of sabotaging a key nuclear installation.
The disclosure came soon before the resumption of talks in Vienna aimed at reviving Iran's 2015 nuclear deal with major powers, an accord Israel fiercely opposed, after former US President Donald Trump abandoned it three years ago.
A fissile purity of 90 percent is suitable for a nuclear bomb.
Announcing 60 percent enrichment, chief nuclear negotiator Abbas Araqchi also said Iran would activate 1,000 advanced centrifuge machines at Natanz, a nuclear plant hit by an explosion on Sunday that Tehran called an act of sabotage by its arch-foe Israel.
However, an Iranian official told Reuters later that "60 percent enrichment will be in small quantity" only.
"From tonight, practical preparations for 60 percent enrichment will begin in Natanz; 60 percent uranium is used to make a variety of radiopharmaceuticals," the semi-official Fars news agency quoted Iranian nuclear agency spokesman Behrouz Kamalvandi as saying.
The International Atomic Energy Agency, the UN nuclear watchdog, said it had been informed of Iran's decision.