"Iran informed the agency of its intention to enrich uranium at a rate of up to 20 percent in its Fordow underground plant, to comply with a law recently passed by the Iranian parliament," an IAEA spokesperson told AFP.
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 AFP, as Tehran continues to retaliate to US sanctions by progressively abandoning limits on its nuclear activity laid down in the deal.
Read also: Iran rejects IAEA chief's call for new nuclear agreement after Biden takes office
According to the latest report available from the UN agency, published in November, Tehran was enriching uranium to levels greater than the limit provided for in the Vienna agreement (3.67 percent) but not exceeding the 4.5 percent threshold, and still complied with the Agency's very strict inspection regime.
But there has been turmoil since the assassination in late November of Iranian nuclear physicist Mohsen Fakhrizadeh.
In the aftermath of the attack, blamed on Israel, hardliners in Tehran pledged a response and parliament passed a controversial law calling for the production and storage of "at least 120 kilogrammes per year of 20 percent enriched uranium" and to "put an end" to the IAEA inspections intended to check that the country is not developing an atomic bomb.
The Iranian government opposed the initiative 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.
The Democrat has shown himself to be determined to save the pact.
Biden, who takes office on January 20, has signalled Washington would rejoin the so-called Joint Comprehensive Plan of Action (JCPOA) aimed at limiting Iran's nuclear programme.
The deal has been unravelling ever since President Donald Trump dramatically withdrew from it in May 2018 and imposed crippling economic sanctions on Tehran.
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".
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