Iran has notified the UN's nuclear watchdog that it will enrich uranium up to 20 per cent, a technical step away from weapons-grade levels, as it increases pressure on the West to revive the 2015 atomic deal that freed it from economic sanctions.
"Iran informed the agency of its intention to enrich uranium at a rate of up to 20 per cent in its Fordo underground plant, to comply with a law recently passed by the Iranian parliament," a spokesperson for the Vienna-based International Atomic Energy Agency said.
The letter dated December 31 "did not state exactly when this enrichment activity would begin", the spokesperson added.
Russian ambassador to the IAEA Mikhail Ulyanov reported the information earlier on Twitter, citing a report submitted by IAEA chief Rafael Grossi to the board of governors.
"It is an additional blow," a diplomat based in Vienna told Agence France-Presse, as Tehran continues to retaliate to US sanctions by progressively abandoning limits on its nuclear activity laid down in the deal.
Iran said it would be carrying out the additional enrichment at its underground Fordo nuclear facility, another violation of the terms of its agreement with world powers. Recent satellite photos show Iran has begun construction on a new site at Fordo, near the holy Shiite city of Qom and about 90 kilometres south-west of Tehran. Shielded by mountains, Fordo is ringed by anti-aircraft guns and other fortifications. It is about the size of a football field, large enough to house 3,000 centrifuges.
According to the latest report available from the UN agency, published in November, Tehran was enriching uranium to levels greater than 3.67 per cent the limit provided for in the 2015 agreement, but not exceeding the 4.5 per cent threshold, and was still complying with the IAEA's very strict inspection regime.
Iran's incremental violations of the accord followed US President Donald Trump's decision to withdraw from the pact in May 2018 and imposed crippling economic sanctions on Tehran. The move led to increasing US-Iran tensions that culminated in the assassination of top Iranian general Qassem Suleimani in a US drone strike on January 3 last year.
After the assassination of Iranian nuclear physicist Mohsen Fakhrizadeh near Tehran in late November, which Iran blamed on Israel, Iran's parliament passed a controversial law calling for the production and storage of "at least 120 kilograms per year of 20 per cent enriched uranium" and to "put an end" to the IAEA inspections intended to check that the country is not developing an atom bomb.
The Iranian government opposed the law which was also condemned by the other signatories to the accord who called on Tehran not to "compromise the future".
The other signatories to the deal – China, France, Germany, Russia and Britain – have been playing for time, in advance of Joe Biden's inauguration as US president.
Mr Biden, who takes office on January 20, has signalled that Washington would rejoin the so-called Joint Comprehensive Plan of Action (JCPOA) aimed at limiting Iran's nuclear programme.
German Foreign Minister Heiko Maas has said the change of administration in the US means that there is "a last window" for progress that "shouldn't be wasted".