The International Atomic Energy Agency (IAEA) has urged Syria to cooperate fully over allegations that it had been building a covert nuclear reactor at a remote desert site called Deir Ezzor -- which Israel destroyed in a bombing raid in 2007.

The probe has lasted for years, with the IAEA saying in a 2011 report it was "very likely" that a building at the Deir Ezzor site "was a nuclear reactor which should have been declared" to the agency.

Syria, whose long-time authoritarian leader Bashar al-Assad was overthrown in December last year after 14 years of civil war, has repeatedly denied it was constructing a nuclear reactor.

As part of its investigation into Deir Ezzor, the agency has been probing three unnamed locations "allegedly functionally related" to the site.

The agency was given permission to take environmental samples in 2024, when Assad was still in power.

In a restricted report circulated to board members on Monday, the agency said it had found "a significant number of natural uranium particles in samples taken at one of the three locations".

It added that analysis of the particles "indicated that the uranium is of anthropogenic origin, i.e. that it was produced as a result of chemical processing".

"The current Syrian authorities indicated that they had no information that might explain the presence of such uranium particles," said the confidential report.

The IAEA said Syria granted its inspectors access to the location in June for a second time to gather more samples.

UN nuclear watchdog head Rafael Grossi visited Damascus in early June, in his first visit to the country since Assad was ousted.

During his stay, he said the IAEA and Syrian authorities would begin "exploring the possibility of nuclear power".

Grossi told an IAEA Board of Governors meeting earlier this year that he had requested Syria's cooperation to "fulfil our obligation to verify nuclear material and facilities" and to "address unresolved issues".