Iran has started producing uranium metal, despite being warned by world powers that it constitutes another breach of the 2015 nuclear deal.
International Atomic Energy Agency inspectors verified the existence of 3.6g (0.1oz) of the substance at a facility in Isfahan last week.
Iran says it is carrying out research and development with the aim of producing fuel for a research reactor.
But uranium metal could also be used to make the core of a nuclear bomb.
Iran insists that its nuclear programme is entirely peaceful, but it has rolled back a number of commitments under the nuclear deal over the past two years.
It says it is retaliating for the crippling US economic sanctions that were reinstated in 2018 by then President Donald Trump when he abandoned the accord.
His successor, Joe Biden, says Iran must return to full compliance before he will lift the sanctions.