Iran said Tuesday it has started uranium enrichment to 60% at the reopened underground facility of Fordo.
The plant was revived three years ago amid the breakdown of its nuclear deal with major global powers.
The move was part of Iran's response to the U.N. nuclear watchdog's adoption last week of a censure motion drafted by Western governments accusing it of non-cooperation.
"Iran has started producing uranium enriched to 60% at the Fordo plant for the first time," Iran's ISNA news agency reported, a development then confirmed by Atomic Energy Organisation of Iran chief Mohammad Eslami.
An atomic bomb requires uranium enriched to 90%, so 60% is a significant step toward weapons-grade enrichment.
Iran has always denied any ambition to develop an atomic bomb, insisting its nuclear activities are for civilian purposes only.
Under a landmark deal struck in 2015, Iran agreed to mothball the Fordo plant and limit its enrichment of uranium to 3.67%, sufficient for most civilian uses, as part of a package of restrictions on its nuclear activities aimed at preventing it from covertly developing a nuclear weapon.
In return, major powers agreed to relax the sanctions they had imposed on Iran's nuclear program.
But the deal began falling apart in 2018 when then U.S. President Donald Trump pulled Washington out of the agreement and reimposed crippling economic sanctions.
The following year, Iran began stepping away from its commitments under the deal. It reopened the Fordo plant and started enriching uranium to higher levels.
In January 2021, Iran said it was working to enrich uranium to 20% at Fordo. Several months later another Iranian enrichment plant reached 60%.
President Joe Biden has expressed a desire for Washington to return to a revived deal and on-off talks have been underway since April last year.
Secretary of State Antony Blinken said late last month that he saw little scope to restore the deal, as Iran battles nationwide protests sparked by the September death in morality police custody of Mahsa Amini, a 22-year-old Kurdish woman.
The heavily protected Fordo plant around 180 kilometers (110 miles) south of Tehran was built deep underground in a bid to shield it from air or missile strikes by Iran's enemies.
Archfoe Israel has never ruled out military action if it deems it necessary to prevent Iran from developing a nuclear weapons capability.
Israel is widely suspected to hold the region's sole if undeclared nuclear arsenal, although it has consistently refused to confirm or deny that it is nuclear-armed.
The implementation of the 2015 deal was overseen by the International Atomic Energy Agency but the U.N. watchdog's relations with Iran have declined sharply in recent months.
The IAEA board of governors passed a resolution on Thursday criticizing Iran for its lack of cooperation.
"We warned that political pressure and resolutions wouldn't change anything and that the adoption of a resolution would draw a serious response," said Eslami.
"That's why the production of uranium enriched to 60% began at Fordo from Monday."
The ISNA news agency said that the step at Fordo was one part of Iran's response.
"As well, in a second action in response to the resolution, Iran injected (uranium hexafluoride) gas into two IR-2m and IR-4 cascades at the Natanz plant," it said, referring to an older enrichment facility.
The U.N. watchdog has been pressing Iran to explain the discovery of traces of nuclear material at three sites it had not declared, a key sticking point that led to the adoption of an earlier censure motion by the IAEA in June.
In a report seen by AFP earlier this month, the IAEA said Iran's stockpile of enriched uranium stood at 3,673.7 kilograms as of Oct. 22, a decrease of 267.2 kilograms from the last quarterly report.
This included significant stockpiles of uranium enriched to higher levels – 386.4 kilograms to 20% and 62.3 kilograms to 60%.
The IAEA complains that the ability of its inspectors to monitor Iran's stepped-up nuclear activities has been hampered by restrictions imposed by Iran.
