Iranian media reported Wednesday that Iran has put a container containing 2,000 kilograms of uranium hexafluoride in its Fordow nuclear facility in order to begin injecting uranium gas into centrifuges.
The move is Iran's latest step away from the agreement it signed in 2015 with a group of world powers to limit its nuclear activity in exchange for sanctions relief.
Under the Joint Comprehensive Plan of Action, Iran was allowed to keep 1,044 centrifuges at Fordow in six cascades, four of which were to remain idle while the other two were allowed to spin without uranium.
"Iran's 4th step in reducing its commitments under the JCPOA by injecting gas to 1044 centrifuges begins today," Iranian President Hassan Rouhani wrote on Twitter. "Thanks to U.S. policy and its allies, Fordow will soon be back to full operation."
Reuters quoted a spokesman from the U.N.'s nuclear watchdog agency saying its inspectors were on the ground in Iran and would report "any relevant activities" to its headquarters in Vienna.
The United States has criticized Iran's increased nuclear activity, which followed last year's U.S. withdrawal from the nuclear deal and a subsequent push by Iran for the remaining signatories to help Iran deal with U.S. sanctions.
U.S. State Department spokesperson Morgan Ortagus said Tuesday that Iran's actions are a "transparent attempt at nuclear extortion."
"We have made clear that Iran's expansion of uranium enrichment activities in defiance of key nuclear commitments is a big step in the wrong direction, and underscores the continuing challenge Iran poses to international peace and security," Ortagus said in a statement. "The JCPOA was a flawed deal because it did not permanently address our concerns with respect to Iran's nuclear program and destabilizing conduct."
Iran previously went past limits on the amount of enriched material it is allowed to stockpile and the level to which it is allowed to enrich uranium.
Rouhani said in a televised address Tuesday that all the steps Iran has taken so far are reversible if the other parties to the nuclear deal uphold their commitments to provide Iran with relief from economic sanctions.