The State Department said the United States has seen the latest report to members by the International Atomic Energy Agency and believed that Iran "has no credible need to produce uranium metal."

Iran promised not to produce uranium metal, which can be used to build a nuclear bomb, as part of the 2015 Joint Comprehensive Plan of Action (JCPOA) under which it was to scale back its nuclear program drastically in return for sanctions relief.

But Iran said earlier this year that it was researching uranium metal to provide advanced fuel for a research reactor -- one of a series of steps it took outside the JCPOA after former president Donald Trump withdrew the United States from the deal.

"We have made clear that continued nuclear escalations beyond JCPOA limits are unconstructive and inconsistent with a return to mutual compliance," State Department spokesman Ned Price said.

"Iran should cease its nuclear escalations and return to negotiations toward full implementation of the JCPOA in good faith."

President Joe Biden favors a return to the deal, believing it peacefully manages a major concern with Iran, but indirect talks in Vienna brokered by the Europeans made no breakthrough with the US administration refusing to lift sanctions unrelated to the nuclear issue.

The talks have been at a standstill since the inauguration of Iran's new ultraconservative president, Ebrahim Raisi, although he says he supports efforts to lift US sanctions.