As announced earlier this week, Iran resumed uranium enrichment at its underground Fordow nuclear facility by injecting uranium gas into centrifuges, the country's Atomic Energy Organization (AEOI) said early Thursday local time.
"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) U.N. nuclear watchdog," Tasnim news agencyreported, quoting the AEOI's statement.
The 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 an active nuclear site.
"The process will take a few hours to stabilize and by Saturday, when International Atomic Energy Agency inspectors will again visit the site, a uranium enrichment level of 4.5% will have been achieved," AEOI's spokesman Behrouz Kamalvandi told state TV. Ninety percent purity is required for bomb-grade fuel.
Iran has gradually scaled back its commitments to the Joint Comprehensive Plan of Actions or JCPOA, as the nuclear agreement is known.
The United States withdrew from the accord last year and reimposed sanctions on Iran, demanding to strengthen the agreement and also for Iran to end its aggressive behavior in the region.
But Iran has refused to re-negotiate JCPOA and has said that if sanctions persist, it will gradually break its commitments under the deal and will go beyond permitted limits of enrichment.
The measure will further complicate the chances of saving the accord, which European powers have called on Iran to respect.