"The agency is deeply concerned that undeclared nuclear material may have been present at this undeclared location and that such nuclear material remains unreported by Iran under its safeguards agreement," a report from the International Atomic Energy Agency seen by AFP said.

"After 18 months, Iran has not provided the necessary, full and technically credible explanation for the presence of the nuclear material particles," the report said.

The site in question is in the Turquzabad district of Tehran, previously identified by Israel as an alleged site of secret atomic activity.

Sources say there is no indication the site has been used for processing uranium, but that it could have been used for storing it as late as the end of 2018.

In a separate report also issued on Tuesday, the IAEA said that Iran's stockpile of enriched uranium is now more than 14 times over the limit set down in its 2015 deal with world powers.

The report said that, as of February 16, Iran's total enriched uranium stockpile was 2,967.8 kilogrammes.

The limit in the 2015 deal was set at 300 kilos (660 pounds) of enriched uranium in a particular compound form, which is the equivalent of 202.8 kilos of uranium in non-compound form.

The latest IAEA reports come on the day that Iran began to restrict some site inspections by the IAEA in response to the US refusal to lift sanctions.