(Bloomberg) -- Global atomic monitors reported Iran’s stockpile of highly-enriched uranium swelled to a record in the last three months and that inspectors are still trying to clarify how uranium enriched to just below weapons-grade was produced at a nuclear facility. 

The International Atomic Energy Agency report is the latest warning about a rapidly-expanding Iranian nuclear program that Central Intelligence Director Bill Burns says has grown more worrisome. Uranium particles enriched to 84% purity were detected at the end of January during an inspection of its Fordow Fuel Enrichment Plant, according to the IAEA. 

“The agency informed that these findings were inconsistent with the level of enrichment” officially declared by Iran, read the 14-page restricted report seen by Bloomberg. “Iran informed the agency that ‘unintended fluctuations in enrichment levels may have occurred.’ Discussions between the the agency and Iran to clarify the matter are ongoing.”

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“These events clearly indicate the capability of the agency to detect and report in a timely manner changes in the operation of nuclear facilities in Iran,” Rafael Mariano Grossi, director-general of the IAEA, said. 

Iran’s stockpile of uranium enriched to 60% levels of purity rose to 87.5 kilograms (193 pounds) from 62.3 kilograms. Inventories of 20%-enriched material rose to 435 kilograms from 386 kilograms.

It’s the ninth consecutive quarter that Iranian engineers increased the country’s volume of nuclear fuel enriched to just below the grade needed for weapons, despite suffocating US sanctions. 

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Tuesday’s report comes amid heightened tensions between Iran and the West over several overlapping crises including an effective suspension of nuclear talks with world powers, Tehran’s military support for Russia’s war on Ukraine and the Islamic Republic’s deadly crackdown of widespread anti-government protests.

On Monday, a senior Iranian official said that a visit by IAEA inspectors, including Grossi, was expected in the “coming days.” The government has so far dismissed concerns about its enrichment levels and said traces of 84%-enriched uranium amounted to a “technical matter.”

The UN monitors reiterated their warning that the longer Iran goes without its nuclear deal brokered by world powers, which capped its atomic work before it unraveled after the US withdrew from it in 2018, the harder it will be for the IAEA to quickly re-establish compliance.

“Any future baseline for the JCPOA verification and monitoring activities would take a considerable time to establish and would have a significant degree of uncertainty,” inspectors wrote, referring to the 2015 nuclear accord using its official acronym. 

The agency would be prepared to work on specific arrangements with Iran, which would be indispensable to addressing this issue, the report said.

While agency monitors have full access to Iran’s uranium enrichment facilities, they’ve lost sight over places where the fast-spinning machines that separate the metal is made. That absence of oversight leaves open the possibility that Iran could be diverting centrifuges to an undeclared location.

(Updates with more quotes from report and background.)

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