As Iran and world powers appear closer than ever to signing a nuclear deal, a report Wednesday said that Tehran has moved to its next level of enriching uranium on new advanced centrifuges.

According to the report from the International Atomic Energy Agency seen by Reuters, Iran has started enriching uranium on the second of three clusters of advanced IR-6 centrifuges at its underground Natanz nuclear plant.

IR-6 centrifuges, the country’s most advanced model, can enrich uranium to at least 60 percent. Iran has been using its existing devices at an aboveground site in Natanz for that purpose for nearly a year. Uranium enriched to 60% is nearly weapons-grade. The new centrifuges are far more efficient, and can more easily switch between enrichment levels.

On Monday, an IAEA report seen by Reuters verified that Iran had begun feeding uranium hexafluoride (UF6) gas into its first cluster of IR-6 centrifuges. Under the terms of its 2015 nuclear agreement with world power, Iran is only permitted to enrich uranium to 3.67% purity. Reuters said the two clusters of new centrifuges are now enriching uranium to 5% purity.

A purported copy of the EU draft deal sent to the US and Iran earlier this month would see Iran freeze its uranium enrichment, although it will be allowed to hold on to the enriched uranium it will have stockpiled before that date.

An official with knowledge of the negotiations told Reuters last week that under the emerging deal, Iran would not be allowed to possess uranium at 20% and 60% enrichment levels and would be forced to switch off its advanced centrifuges, preventing it from amassing the material required to build a bomb.

EU and US officials both said Wednesday that significant progress was being made toward returning to the 2015 nuclear deal with Iran, even as early as next week.

But in recent weeks the activities of the IAEA have emerged as a sticking point, with Iranian officials proclaiming that a deal would not be reached if the UN nuclear watchdog did not halt an ongoing probe — something the US opposes.

On Wednesday, Iranian Foreign Minister Hossein Amir-Abdollahian, visiting Moscow, called for stronger US guarantees and an end to investigations into Iranian nuclear activity by the IAEA. Amir-Abdollahian echoed similar comments made earlier this week by Iranian President Ebrahim Raisi, who said there will be no deal if IAEA inspectors continue their probe.

“Without settlement of safeguard issues, speaking about an agreement has no meaning,” Raisi said.

The IAEA for years has sought for Iran to answer questions about human-made uranium particles found at undeclared sites. US intelligence agencies, Western nations and the IAEA have said Iran ran an organized nuclear weapons program until 2003. Iran long has denied ever seeking nuclear weapons.

A spokesman for the US State Department said earlier this week that the White House does not intend to curtail the IAEA’s activities as part of a return to the 2015 Joint Comprehensive Plan of Action.

“Safeguards on nuclear materials relate to the very core of the IAEA’s mandate,” Vedant Patel said. “And we have been crystal clear that we do not believe there should be any conditionality between reimplementation of the JCPOA and the investigations related to Iran’s legal obligations under the NPT and its Comprehensive Safeguard Agreement.”

Israel has heavily lobbied to stop the US from acceding to Iran’s demand to halt the IAEA probe. Prime Minister Yair Lapid and US President Joe Biden spoke via phone on Wednesday about the nuclear deal, and Biden reportedly assured Lapid that the White House will not give in to Tehran on the issue.

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