Iran has started enriching uranium at its underground Natanz plant with a second type of advanced centrifuge, the IR-4, according to a report by the UN nuclear watchdog, marking a further breach of Tehran's deal with major powers.

"On March 15, 2021, the Agency verified that Iran began feeding the cascade of 174 IR-4 centrifuges already installed at FEP (Fuel Enrichment Plant) with natural UF6," the International Atomic Energy Agency (IAEA) said in the report to member states dated Monday, referring to uranium hexafluoride, the form in which uranium is fed into centrifuges for enrichment.

Iran has indicated that it now plans to install a second cascade of IR-4 centrifuges at the FEP but installation of that cascade has yet to begin, the report said. Iran has already increased the number of IR-2m machines, which are far more efficient than the IR-1, installed at the underground plant.

"In summary, as of March 15, 2021, Iran was using 5,060 IR-1 centrifuges installed in 30 cascades, 522 IR-2m centrifuges installed in three cascades and 174 IR-4 centrifuges installed in one cascade, to enrich natural UF6 up to 5 percent U-235 at FEP," the IAEA report said, referring to the fissile purity of uranium.

Earlier this year, the Iranian government said it had resumed 20 percent uranium enrichment at another nuclear facility, Fordow.

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The news regarding the IR-4 centrifuges came after the IAEA chief told European lawmakers that a U.S. return to the 2015 deal with Iran remains possible, and Iran's Foreign Minister Mohammad Javad Zarif in early March said that Tehran will offer a "constructive" plan amid hopes of informal talks with major powers including the United States.

Iran has recently accelerated its breaches of the deal's restrictions on its nuclear activities in a bid to pressure U.S. President Joe Biden as both sides are locked in a standoff over who should move first to save the deal.

Tehran's breaches began in 2019 in response to the U.S. withdrawal from the deal and the reimposition of U.S. economic sanctions against Iran under former President Donald Trump.

Last year, Iran started moving three clusters of different advanced models of centrifuges from an above-ground plant at Natanz to its below-ground FEP. It is already enriching underground with IR-2m centrifuges. The deal only lets it enrich there with first-generation IR-1 machines.

(With input from Reuters)