The UN's nuclear watchdog has asked Iran for details on why traces of uranium were found at an undeclared site after Iranian officials failed to provided information about the discovery.
The issue has added to tensions over Iran's moves to breach a landmark 2015 deal aimed at curbing its nuclear activities.
"It is essential that Iran works with the agency to resolve this matter promptly," Cornel Feruta told the International Atomic Energy Agency's (IAEA) board of governors in Vienna on November 21.
The IAEA said in a report made public last week that its inspectors had "detected natural uranium particles of anthropogenic origin at a location in Iran not declared to the agency."
"We have continued our interactions with Iran since then, but have not received any additional information and the matter remains unresolved," Feruta said.
The IAEA has not named the site in question, but inspectors are believed to have taken samples from a location in Tehran's Turquzabad district where Israel has alleged secret atomic activity in the past.
The discovery of uranium particles of man-made origin at a location that has not been declared comes as the 2015 nuclear deal between Iran and world powers threatens to collapse.
The United States in May last year pulled out of the agreement and started to reinstate punishing sanctions on Tehran, leaving the other signatories struggling to salvage the accord.
Over the past few months Iran has breached several parts of the pact, under which it committed to scale back its nuclear program in exchange for sanctions relief.
On November 18, the IAEA confirmed that Iran's stock of heavy water for reactors had surpassed the 130-ton limit set under the nuclear deal.