The Vienna-based International Atomic Energy Agency (IAEA) has for years been calling on Iran to explain the presence of undeclared man-made uranium found at three sites, requesting "access to locations and material" as well as the collection of samples.

"At the invitation of Iran, an IAEA technical team will be in Tehran on Sunday", an IAEA spokeswoman said.

The December 18 visit is aimed at "addressing the outstanding safeguards issues previously reported", she added.

UN nuclear watchdog chief Rafael Grossi is not expected to visit Iran this time.

Earlier Wednesday, Iran had said that IAEA officials would visit the country to settle "ambiguities" over claims of secret activities.

The issue has increasingly frustrated efforts to revive the 2015 Iran nuclear deal, that has been on life support since the United States unilaterally withdrew from it in 2018 under then-president Donald Trump.

"Our interactions with the agency are ongoing, and we hope that we can make effective progress with the agency in order to resolve the obstacles and ambiguities," Mohammad Eslami, the head of the Atomic Energy Organization of Iran, told reporters in Tehran.

An IAEA delegation had initially planned to travel to Tehran last month, but the visit did not take place after the agency's board of governors deplored Iran's lack of cooperation in providing "technically credible" answers.

In the absence of progress, the agency says it cannot guarantee the authenticity and integrity of Iran's nuclear programme.

Last Friday, Eslami said traces of enriched uranium found in Iran were brought into the country from abroad.

The 2015 agreement gave Iran sanctions relief in exchange for curbs on its nuclear programme to guarantee that Tehran could not develop a nuclear weapon -- something it has always denied seeking to do.