Iran has resumed uranium enrichment at its underground Fordow nuclear facility by injecting uranium gas into centrifuges, Iran's Tasnim news agency reports.
"After all successful preparations ... injection of uranium gas to centrifuges started on Thursday at Fordow ... all the process has been supervised by the inspectors of (the) UN nuclear watchdog," Tasnim reported on Thursday.
It was quoting a statement from the country's Atomic Energy Organisation (AEOI).
A 2015 nuclear deal between Iran and major powers bans Fordow from producing nuclear material.
But, with feedstock gas entering its centrifuges, the facility - built inside a mountain - will move from the permitted status of research plant to being an active nuclear site.
"The process will take a few hours to stabilise and by Saturday, when International Atomic Energy Agency inspectors will again visit the site, a uranium enrichment level of 4.5 per cent have been achieved," AEOI's spokesman Behrouz Kamalvandi told state TV.
Ninety per cent purity is required for bomb-grade fuel.
Iran has gradually scaled back its commitments to the deal, under which it curbed its nuclear program in exchange for the removal of most international sanctions, after the United States reneged on the agreement last year.
Under the pact, Iran agreed to turn Fordow into a "nuclear, physics and technology centre" where 1,044 centrifuges are used for purposes other than enrichment, such as producing stable isotopes, which have a variety of civil uses.
"All the centrifuges installed in Fordow are IR1 types. Uranium gas (UF6) was injected to four chains of IR1 centrifuges (696 centrifuges)," Kamalvandi said.
"Two other remaining chains of IR1 centrifuges (348 centrifuges) will be used for producing and enriching stable isotopes in the facility."
US President Donald Trump exited the deal, saying it was flawed to Iran's advantage.
Washington has since renewed and intensified sanctions on Iran, slashing the country's economically vital crude oil sales by more than 80 per cent.
The measure will further complicate the chances of saving the accord, which European powers have called on Iran to respect.
Responding to Washington's "maximum pressure" policy, Iran has bypassed the restrictions of the deal step-by-step - including by breaching both its cap on stockpiled enriched uranium and on the fissile level of enrichment.
Australian Associated Press