The International Atomic Energy Agency (IAEA) on Thursday urged Iran to explain the presence of uranium particles at an undeclared site, AFP reported.
The call comes a week after the UN nuclear watchdog said in a report that its inspectors had "detected natural uranium particles of anthropogenic origin at a location in Iran not declared to the agency".
The IAEA has not named the site in question, though sources say it is located in the Turquzabad district of Tehran. It is believed to be the site that was identified by Prime Minister Binyamin Netanyahu during his address before the UN General Assembly last year.
Reports in April indicated that the IAEA had inspected the secret atomic warehouse. Subsequent reports said the facility was found to contain traces of uranium.
The agency's acting head Cornel Feruta said IAEA and Iranian officials would meet in Tehran next week to discuss the matter, adding that the UN body had not received any additional information.
"The matter remains unresolved... It is essential that Iran works with the agency to resolve this matter promptly," he told IAEA member states at a meeting of the agency's board of governors, according to AFP.
A diplomatic source told AFP that the IAEA would send a high-ranking technical delegation to Iran next week.
The particles are understood to be the product of uranium which has been mined and undergone initial processing, but not enriched.
The IAEA also recently confirmed that Iran breached another limit of its 2015 nuclear deal with major powers by accumulating slightly more than 130 tons of heavy water.
Iran has gradually scaled back its compliance with the 2015 deal in response to US President Donald Trump’s withdrawal from the agreement last May.
Before the heavy water breach, the Islamic Republic restarted uranium enrichment at the underground Fordow facility in violation of the deal.
Britain, France and Germany have been trying to save the nuclear deal and have vowed to help Iran evade the economic sanctions imposed by the US, shielding companies doing business with the rogue state in an effort to preserve the Iran nuclear deal.
The chief of Iran's atomic agency recently blamed Europe for his country scaling back its commitments under the deal, saying their broken promises gave the Islamic Republic little choice.
