Iran has informed the United Nations nuclear watchdog that it intends to start enriching uranium to 20 percent purity, marking its most significant breach of the 2015 Vienna international nuclear deal so far.

Though the purity remains short of the 90 percent required to make a nuclear bomb, it is much higher than the standard set in the 2015 agreement, under which Iran was supposed to keep enrichment below four percent. 

"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," said a spokesperson of International Atomic Energy Agency (IAEA).

A letter from Iran 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," said a diplomat based in Vienna, as Tehran continues to retaliate against U.S. sanctions by progressively abandoning limits on its nuclear activity laid down in the deal.

According to the latest report available from the UN agency, published in November, Tehran was enriching uranium to levels higher than the limit stipulated in the Vienna agreement (3.67 percent) but not exceeding the 4.5 percent threshold and still complying 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 its parliament passed a controversial law calling for the production and storage of "at least 120 kilograms 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."

Joe Biden, who is scheduled to take office as U.S. president on January 20, has signaled Washington would rejoin the so-called Joint Comprehensive Plan of Action (JCPOA) aimed at limiting Iran's nuclear program.

As for Iran, it has expressed interest in once again complying with the agreement if the U.S. returns to the deal.

The deal has been unraveling ever since President Donald Trump dramatically withdrew from it in May 2018 and imposed crippling economic sanctions on Tehran.

(With input from AFP)