Rafael Grossi, the new head of the International Atomic Energy Agency (IAEA), who was in Paris to meet President Emmanuel Macron, told AFP: "Iran must decide to cooperate in a clearer manner with the agency to give the necessary clarifications."

"The fact that we found traces (of uranium) is very important. That means there is the possibility of nuclear activities and material that are not under international supervision and about which we know not the origin or the intent.

"That worries me," Grossi added.

The IAEA has for months been pressing Tehran for information about the kind of activities being carried out at the undeclared site where the uranium particles were found.

While the IAEA has not identified the site in question, diplomatic sources told AFP the agency asked Iran about a site in the Turquzabad district of Tehran, where Israel has alleged secret atomic activity in the past.

His warning came as the agency released a report on Iran's nuclear activities since it began breaching several parts of a landmark 2015 international deal on scaling back its nuclear programme.

The report showed its stockpile of enriched uranium now stands at more than five times the limit fixed under the deal with world powers.

Iran began reneging on its commitments after the US pulled out of an agreement which offered sanctions relief in return.