The UN's Vienna-based International Atomic Energy Agency, or IAEA, has for months been calling on Iran to explain the presence of nuclear material at three undeclared sites.

The issue has 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.

"Agency officials will visit Tehran in the coming days," Mohammad Eslami, the head of the Atomic Energy Organization of Iran, told reporters in Tehran.

"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 and take a step forward," he added.

An IAEA delegation had 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.

As a result, the agency said it was unable to guarantee the authenticity and integrity of Iran's nuclear programme.

On 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.