The global nuclear watchdog has found uranium particles enriched to 83.7% purity – very close to weapons grade – at Iran’s underground Fordo site.
In a report seen by the BBC, the International Atomic Energy Agency (IAEA) said it was having discussions with Iran “to clarify the matter”.
Iran has said “unintended fluctuations” in enrichment levels may have occurred.
It has been openly enriching uranium to 60% purity for two years in breach of a 2015 nuclear deal with world powers.
The agreement, which was aimed at preventing Iran from developing nuclear weapons, saw the country limit its nuclear activities and allow monitoring by the IAEA’s inspectors in return for relief from crippling economic sanctions.
However, it has been close to collapse since then US-President Donald Trump pulled out unilaterally and reinstated sanctions in 2018 and Iran retaliated by increasingly breaching the restrictions.
Joe Biden’s administration wants to rejoin the deal if Iran returns to compliance, but indirect negotiations in Vienna, where the IAEA is based, have been stalled for a year.
Uranium is a naturally-occurring element that can have nuclear-related uses once it has been refined, or enriched. This is achieved by using centrifuges – machines which spin at supersonic speeds – to separate out the most suitable isotope for nuclear fission, called U-235.
Low-enriched uranium, which typically has a 3-5% concentration of U-235, can be used to produce fuel for commercial nuclear power plants.
Highly enriched uranium has a purity of 20% or more and is used in research reactors. Weapons-grade uranium is 90% enriched or more.