A near-complete nuclear reactor built secretly during the rule of toppled Syrian leader Bashar al-Assad would have been able to produce material to make nuclear weapons, the UN nuclear watchdog chief said on Monday.

Inspectors from the International Atomic Energy Agency (IAEA) last month visited two sites, one in Deir Ezzor and a second undisclosed location identified to them by Syrian authorities.

In a report seen by AFP last week, the IAEA said it found that Syria's previous government was building a nuclear reactor.

Assad was toppled in December 2024, and the current Islamist government came to power.

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IAEA head Rafael Grossi said it was "obvious that the previous government was pursuing a number of clandestine" activities.

"They didn't want the world to know what they were doing," he told reporters after opening the Vienna-based agency's quarterly board of governors meeting.

"Given the configuration, the technical features of that type of reactor, this is a reactor that is very efficient in the production of fissile material that could be used directly for the fabrication of nuclear weapons," he said, adding that building the reactor was a "very important step."

He said the reactor – which is currently being excavated – "was almost completed."

It was "very possible... that the then Syrian authorities also performed some explosions in order to... hinder any possibility of an investigation," he added.