The UN's International Atomic Energy Agency, or IAEA, has for months been pressing Tehran to explain the presence of the nuclear material at three undeclared sites.

The discovery further complicated efforts to revive the 2015 Iran nuclear deal that has been hanging by a thread since the United States unilaterally withdrew from it in 2018 under then-president Donald Trump.

In remarks published Thursday by Hamshahri newspaper, the head of the Atomic Energy Organization of Iran, Mohammad Eslami, said the traces came from waste brought into Iran from other countries.

Eslami said the places visited by UN inspectors were a cattle farm, an abandoned mine and a landfill.

"In the landfill, they took samples from the waste that entered Iran from different countries," the report quoted him as saying.

"This does not mean the place of discovery was a nuclear site or that it was an undeclared nuclear activity."

"The waste came from Iraq and from other countries," Eslami said.

"We have prevented the entry of much of this waste... They were not nuclear substances from our own manufacturing but perhaps traces from previous use in the country of origin."

In a resolution last month, the IAEA's board of governors deplored the lack of cooperation an "technically credible" answers from Tehran.

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

But Eslami said Tehran has "provided documented and argued answers to the request" of the UN nuclear watchdog.

An IAEA delegation had planned to travel to Tehran in November, but the visit did not take place.

The 2015 deal was designed to prevent Iran from covertly developing a nuclear bomb, a goal the Islamic republic has always denied.

Efforts to get Iran and the United States back on board with the agreement have stalled.