Syria’s ousted Assad regime was building a nuclear reactor designed to produce fissile material that could potentially have been used for nuclear weapons, the head of the U.N. nuclear watchdog said 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.

Syria was believed to have operated an extensive undeclared nuclear program under Assad that included a nuclear reactor built by North Korea in eastern Deir el-Zour province.

Following Assad’s ouster, the new government agreed to give IAEA inspectors access to suspected former nuclear sites.