"On 31 March 2021, the Agency verified at FEP that: Iran had begun feeding natural UF6 into a fourth cascade of 174 IR-2m centrifuges," the International Atomic Energy Agency said in its confidential report dated Wednesday and obtained by Reuters on Thursday. By UF6, it was referring to uranium hexafluoride, the form in which uranium is fed into centrifuges for enrichment.

It was the latest of many steps by Iran raising pressure on U.S. President Joe Biden with the two sides in a standoff over who should move first to salvage a deal that was meant to curb Iran's ability to develop a nuclear bomb, if it so intended.

Efforts to sketch out initial U.S. and Iranian steps to resume compliance with the 2015 nuclear deal are sought to be commenced from the U.S. side and Western officials believe Iran is not in rush to discuss a wider road map to revive the pact, something Washington is willing to do.