Iran has started enriching uranium with a third set of advanced IR-2m centrifuges at its underground plant at Natanz, the UN nuclear watchdog told its member states today, a further breach of Tehran's 2015 deal with major powers.

The move is part of a recent acceleration by Iran of its violations of restrictions under that deal, which granted Iran relief from financial sanctions in return for curbs to its nuclear activities.

It began breaching limits after then-US president Donald Trump quit the deal and re-imposed sanctions in 2018.

The acceleration of breaches appears aimed at raising pressure on Mr Trump's successor Joe Biden. The new US president wants to revive the accord, but Washington and Tehran are locked in a standoff over which side should move first.

The deal allows Iran to enrich uranium only with first-generation IR-1 centrifuges at the underground, commercial scale Fuel Enrichment Plant (FEP) at Natanz. In November Iran started enriching there with a first set of IR-2m machines, which are far more efficient, and has since been adding to it.

"On 7 March 2021, the Agency verified at FEP that: Iran had begun feeding natural UF6 into the third cascade of 174 IR-2m centrifuges," the International Atomic Energy Agency said in a report obtained by Reuters, referring to uranium hexafluoride, the form in which uranium is fed into centrifuges to purify it.

"The fourth cascade of 174 IR-2m centrifuges was installed but had yet to be fed with natural UF6; installation of a fifth cascade of IR-2m centrifuges was ongoing; and installation of a sixth cascade of IR-2m centrifuges had yet to begin," it added.

In addition to its IR-1 machines, Iran is now using 522IR-2m centrifuges to enrich uranium to up to 5% fissile purity at the FEP, the IAEA added. That is more than the 3.67% purity allowed under the deal but less than the 20% it is enriching at another facility, Fordow. Uranium enriched to 90% purity can be used to make an atomic weapon.