The visit comes after a report by Bloomberg News last week, strongly denied by Tehran, that its nuclear scientists have enriched uranium to 84 percent purity, close to the 90 percent threshold required to produce an atomic bomb.

"Agency officials are in Tehran and have been negotiating, visiting and investigating since yesterday (Tuesday)," Atomic Energy Organization of Iran (AEOI) chief Mohamad Eslami told reporters on the sidelines of a weekly cabinet meeting.

"The ambiguities that were created by the opinion of an inspector have been resolved or are in the process of being resolved."

The Vienna-based IAEA said Sunday that it was in discussions with Tehran following the Bloomberg News report which drew an angry response from Iranian officials.

AEOI spokesman Behrouz Kamalvandi described the report as "slander" and a "distortion of the facts".

"The presence of a particle or particles of uranium above 60 percent in the enrichment process does not mean enrichment above 60 percent," he said.

The latest visit by IAEA inspectors comes amid deadlock in negotiations on reviving a 2015 deal between Iran and major powers, that promised relief from crippling economic sanctions in exchange for curbs on Iran's nuclear activities.

The curbs, which included a 3.67 percent on uranium enrichment, were intended to prevent Iran developing a nuclear weapons capability, an ambition it has always denied.

But the United States unilaterally withdrew from the deal in 2018 and reimposed sanctions, prompting Iran to suspend the implementation of its own commitments under the accord.