The Atomic Energy Organization of Iran (AEOI) confirmed late on Wednesday that Iran resumed uranium enrichment at its underground Fordow nuclear facility by injecting uranium gas into centrifuges.

“After all successful preparations ... injection of uranium gas to centrifuges started on Thursday (local time) at Fordow ... all the process has been supervised by the inspectors of (the) UN nuclear watchdog,” the Tasnim news agency reported, quoting the AEOI’s statement.

“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.

The statement confirms earlier reports on Iranian state TV that Iran had renewed uranium enrichment activities at the underground Fordow facility, located outside of Qom.

Iranian President Hassan Rouhani announced on Tuesday that Iran would move forward with the injection of uranium gas into 1,044 centrifuges at the Fordow site, the latest violation of Iran's commitments under the 2015 nuclear deal.

Later on Tuesday, Iran announced that it would enrich uranium to 5% at the Fordow nuclear facility, in a further violation of the 2015 nuclear deal it signed with world powers.

US President Donald Trump withdrew from the 2015 deal last May. Since that time, his administration has ramped up the sanctions on the Islamic Republic.

In response to the US withdrawal from the nuclear deal and its imposition of sanctions, Iran has gradually scaled back its compliance with the 2015 deal.

The European Union on Tuesday voiced concern following Iran’s announcement that it would resume uranium enrichment at the Fordow underground plant.

“We are concerned by President Rouhani’s announcement today to further reduce Iran’s commitments under the JCPOA,” EU spokeswoman Maja Kocijancic told reporters.

“We urge Iran to reverse all activities that are inconsistent with its commitments under the JCPOA and to refrain from any further measures that would undermine the preservation and full implementation of the nuclear deal,” she added.

The US State Department, meanwhile, said Iran’s move is “a big step in the wrong direction”.

“We fully support the International Atomic Energy Agency (IAEA) in conducting its independent verification role in Iran and look to the IAEA to report on any developments,” said a State Department spokesperson.