Iran says fluctuations at its Natanz nuclear plant pushed the purity to which it enriched uranium to 63 per cent, higher than the 60 per cent it announced last month, a report by the UN nuclear watchdog said on Tuesday.
Iran in April made the shift to 60 per cent, a big step towards nuclear-weapons grade from the 20 per cent previously achieved.
That came after an explosion and power cut at Natanz, which appears to have damaged its enrichment output at a larger, underground complex there. Iran blames Israel for the two incidents.
Iran's move threatened indirect talks with the US to agree on conditions for both sides to return fully to the 2015 nuclear deal, which was undermined when Washington abandoned it in 2018.
The deal says Iran cannot enrich beyond 3.67 per cent fissile purity, far from the 90 per cent of weapons grade.
Iran has long denied any ambition to develop nuclear weapons.
"According to Iran, fluctuations of the enrichment levels were experienced," the International Atomic Energy Agency said in a confidential report to its member states, Reuters reported.
"The agency's analysis of the environmental samples taken on April 22, 2021, shows an enrichment level of up to 63 per cent U-235, which is consistent with the fluctuations of the enrichment levels."
The agency did not say why the fluctuations had occurred.
An IAEA report last month said Iran was using one cascade of advanced IR-6 centrifuges to enrich uranium to 60 per cent.
It said it was feeding the depleted uranium from that process into a cascade of IR-4 machines to enrich it to 20 per cent.
Tuesday's report said Tehran was now feeding the tails from the IR-4 cascade into a cascade of 27 IR-5 and 30 IR-6s centrifuges and refining uranium to 5 per cent.
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