The Assad-era government in Syria was constructing a nuclear reactor that was configured to produce fissile material for possible use in nuclear weapons, the head of the International Atomic Energy Agency said on Monday.

The "configuration" and the "technical features" of the nuclear reactor under construction in Syria during the leadership of ousted former President Bashar Assad were "very efficient in the production of fissile material that could be directly used for the fabrication of nuclear weapons," IAEA director general Rafael Grossi said.

"Of course, that was never declared, that was always denied, so, and again, intentions cannot be judged, but that was a very serious proliferation concern that we had, and this has been difficult work," he told reporters on the first day of a regular meeting of the IAEA Board of Governors, in Vienna.