Iran has started enriching uranium to 60 per cent purity at its underground Fordow nuclear site, according to state media, which describes the action as a response to the UN nuclear watchdog's demand for more co-operation from Tehran.
Iran is already enriching uranium to up to 60 per cent purity elsewhere, well below the roughly 90 per cent needed for weapons grade material but above the 20 per cent it produced before a 2015 agreement with major powers to cap enrichment at 3.67 per cent.
"In a letter to the International Atomic Energy Agency (IAEA), Iran has informed the agency that it has started enriching uranium to 60 per cent purity at the Fordow site," the semi-official ISNA news agency reported on Tuesday, adding it was a "strong response" to the agency's latest resolution.
The IAEA 35-nation Board of Governors on Thursday passed a resolution ordering Iran to co-operate urgently with the agency's investigation into uranium traces found at three undeclared sites, diplomats at the closed-door vote said.
Iran's SNN network said Tehran would also build a new set of centrifuges at its Natanz and Fordow nuclear sites.
In June, Reuters reported that Tehran was escalating its uranium enrichment further by preparing to use advanced IR-6 centrifuges at the Fordow site, which can easily switch between enrichment levels.
The IAEA resolution is the second this year targeting Iran over the investigation, which has become an obstacle to talks on reviving the 2015 Iran nuclear deal because Iran has demanded the probe be ended.
Iran's foreign ministry on Monday dismissed the resolution as "politically motivated".