Iran has begun uranium enrichment with new advanced centrifuges at its underground Natanz nuclear site, according to a confidential International Atomic Energy Agency report seen by the Reuters news agency on Monday.

The report said Iran is pressing ahead with its rollout of IR-6 centrifuges at the site. The centrifuges are far more efficient, and can more easily switch between enrichment levels.

IAEA inspectors verified on Sunday that Iran was feeding uranium hexafluoride (UF6) gas, the material centrifuges enrich, into the first of three cascades, or clusters, of IR-6 centrifuges installed at the Natanz underground Fuel Enrichment Plant (FEP), Reuters said, quoting from the confidential IAEA report to member states.

The centrifuges are being used “for the production of UF6 enriched up to 5% U-235” the IAEA said.

Of the remaining two IR-6 cascades, one was engaged in passivation using depleted UF6, a process performed before proper enrichment is started, and the other has not yet been loaded with nuclear material, the UN agency reported according to Reuters.

IR-6 centrifuges, the country’s most advanced model, can enrich uranium to at least 60%. Iran has been using its existing devices at an above-ground site in Natanz for that purpose for nearly a year, Reuters reported. Uranium enriched to 60% is nearly weapons-grade.

In July, Iran announced it was using new IR-6 centrifuges to enrich uranium to 20%. The IAEA reported in June that Iran had 43 kilograms of uranium enriched to 60% purity — a short step to 90%. Nonproliferation experts warn that’s enough fissile material for one nuclear weapon if Iran chose to pursue it.

Under the terms of its 2015 nuclear agreement with world power, Iran is only permitted to enrich uranium to 3.67% purity. The deal gave Iran sanctions relief in return for curbs on its nuclear program to prevent the production of a weapon. However, after the Trump administration pulled the US out of the so-called Joint Comprehensive Plan of Action (JCPOA) in 2018 and reimposed its own sanctions, Iran dropped many of its commitments to the pact and has ramped up uranium enrichment.

The latest development came as talks to revive the JCPOA were said to be closing in on an agreement headed by the European Union as negotiations in Vienna culminated with what the EU said was a final offer to Iran last month. Tehran responded with its own remarks and the US last week gave its input too.

On Sunday, the Haaretz daily cited what it said was a draft of the EU proposal from before the final document was sent to Iranians and which stipulated that under a new agreement Iran would have to stop its uranium enrichment but would be able to keep the material it has already produced.

Israel has piled pressure on Western countries to halt talks on reviving the agreement, warning against the consequences of returning to the accord. Mossad chief David Barnea will travel to Washington next week as part of the efforts to shape the deal.

Iran insists its program is for peaceful purposes, though UN experts and Western intelligence agencies say Iran had an organized military nuclear program through 2003.

Israel has vowed to do whatever is needed to prevent Iran from becoming a nuclear-armed state, including taking military action.

Former Mossad chief Yossi Cohen said Monday that Israel carried out “countless operations” against Iran’s nuclear program when he led the spy agency between 2016-2021.

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