Bloomberg News reported that inspectors of the International Atomic Energy Agency (IAEA) in Iran last week found uranium enriched to 84-percent purity.
Iran was last known to enrich up to 60 percent, while a 90-percent threshold is required for use in a weapon.
The report comes with negotiations stalled to revive a landmark deal over Iran's nuclear programme.
"The IAEA is aware of recent media reports relating to uranium enrichment levels in Iran," the Vienna-based agency wrote on Twitter.
It added that it was "discussing with Iran the results of recent agency verification activities and will inform the IAEA Board of Governors as appropriate".
Iran started stepping up its nuclear activities in 2019, a year after the US under then president Donald Trump pulled out of the landmark deal and reinstated sanctions.
The 2015 accord promised Iran sanctions relief in exchange for cutting back its nuclear programme.
Negotiations between world powers to return to the deal started in 2021 but have stalled since last year.
Bloomberg said "inspectors need to determine whether Iran intentionally produced the material, or whether the concentration was an unintended accumulation.
"It's the second time this month that monitors have detected suspicious enrichment-related activities," it added.
In January, IAEA head Rafael Grossi said Iran had "amassed enough nuclear material for several nuclear weapons -- not one at this point".
In December, Iran said its uranium enrichment capacity had increased to record levels.
Tehran has repeatedly insisted that it is not planning to build a nuclear bomb.
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