France on Monday condemned with "the utmost firmness" Iran's launch of uranium enrichment at a hard-to-bomb mountain plant, labelling it a "grave" violation of international law.
The UN atomic watchdog said earlier in the day that Iran is now enriching uranium to 20 percent at Fordo, a reinforced facility sunk deep under a mountain 150 kilometres (90 miles) southwest of Tehran.
"This is an additional and particularly grave violation by Iran of international law, of six Security Council resolutions" and of 11 resolutions adopted by the UN's International Atomic Energy Agency, said French foreign ministry spokesman Romain Nadal in a statement.
"This new challenge leaves us no choice but to strengthen international sanctions and to adopt, with our European partners and all willing countries, measures of an unprecedented scale and severity."
Iran, which insists its nuclear programme is for exclusively peaceful purposes, has repeatedly said it will not abandon uranium enrichment.
The French spokesman reiterated that the Fordo plant had "been hidden for many years from the international community, until the autumn of 2009".
He added that "the facility's alleged function has changed over time" and that there had never appeared to be an economic rationale for civilian use.
While nuclear energy plants require fuel enriched to 3.5 percent, Iran says the 20-percent enriched uranium is necessary for its Tehran research reactor to make isotopes to treat cancers.
Western powers, however, reject this, believing Iran has been researching ways to develop and deliver nuclear weapons.
"The pretext of using uranium enriched to 20 percent to operate the research reactor in Tehran is in no way credible, given Iran's persistent refusal to consider our offers to provide such fuel," said Nadal.
He added that Iran's increased capacity to produce uranium enriched to more than 3.5 percent brings it "considerably closer" to military use.