Iran's stock of uranium enriched to 60% is now almost three times the roughly 42kg that by the IAEA's definition is theoretically enough, if enriched further, to produce a nuclear bomb.

Uranium enriched up to 90% purity is considered nuclear weapons-grade.

The amount of material, detected by routine screening, was extremely small and has been assessed by experts as posing no threat to the public, police say.

While indirect talks between Iran and the US on reviving the 2015 Iran nuclear deal have stalled, Tehran has increased the number of advanced centrifuges the deal bans it from using to produce enriched uranium.

For more than a year Iran has been using IR-6 centrifuges to enrich uranium to up to 60% purity, close to weapons-grade, at an above-ground plant at Natanz.

The latest violation of the deal, which is aimed at preventing Tehran from developing nuclear weapons, comes as Iran warns that time is running out for Joe Biden's administration to save the agreement.