Iran said on Thursday it had resumed uranium enrichment at its underground Fordow nuclear plant, stepping further away from its 2015 nuclear deal with world powers after the United States pulled out of it.

Axar.az reports citing foreign media.

The pact bans production of nuclear material at Fordow, a highly sensitive site that Iran hid from U.N. non-proliferation inspectors until its exposure in 2009. But with feedstock gas entering its centrifuges, the facility, built inside a mountain to withstand any air strikes, will move from the permitted status of research plant to being an active nuclear site.

“After all successful preparations, injection of uranium gas into centrifuges started on Thursday at Fordow...The whole the process has been supervised by the inspectors of the U.N. nuclear watchdog,” the Atomic Energy Organization of Iran (AEOI) said in a statement carried by state media.

Iran has gradually scaled back its commitments to the deal, under which it restrained its enrichment program in exchange for the removal of most international sanctions, since the United States reneged on the agreement last year.

“The process will take a few hours to stabilize and by Saturday, when International Atomic Energy Agency inspectors will again visit the site, an...enrichment level of 4.5% will have been achieved,” AEOI spokesman Behrouz Kamalvandi told state TV.

Enrichment of uranium to such a low level of fissile purity would be broadly suitable for civilian electricity generation. Ninety-percent purity is required for nuclear bomb fuel.