Iran says it has begun using new advanced centrifuges to enrich uranium - the latest step in reducing its commitments under a 2015 nuclear deal with world powers.
Forty such centrifuges were now operational, said nuclear agency spokesman Behruz Kamalvandi.
Enriched uranium can be used to make reactor fuel but also nuclear weapons.
Iran stopped abiding by two commitments in July in response to sanctions the US reinstated when it abandoned the deal.
President Donald Trump wants to force Iran to negotiate a new agreement that would place indefinite curbs on its nuclear programme and also halt its development of ballistic missiles. But Iran has so far refused.
What has Iran done?
Mr Kamalwandi announced the new measures on Saturday.
He told a news conference Iran's Atomic Energy Agency had activated 20 IR-4 and 20 IR-6 centrifuges - which could enrich high-quality uranium.
New centrifuges would be developed to meet the country's needs, he said.
He added that Iran's steps were reversible - but this depended on other partners returning to their obligations under the deal.
In the meantime, Iran's would continue to allow inspectors from the global nuclear watchdog, the International Atomic Energy Agency (IAEA).
The IAEA confirmed on 1 July that Iran had breached the 300kg (660lb) limit on the amount of enriched uranium it is allowed to stockpile.