Nearly two decades after Israel destroyed a “nuclear facility under construction” in the Deir ez-Zor region of Syria, the country’s secret nuclear program has once again become a controversial issue.

The International Atomic Energy Agency said that Syria, during the tenure of Bashar al-Assad, the former president, had been close to acquiring the capability to produce fissile material that could be used directly in the construction of a nuclear weapon.

Rafael Grossi, the IAEA director general, said at a press conference that the reactor discovered in Deir ez-Zor was almost complete, had high efficiency, and could produce fissile material used to build a nuclear weapon.

Mr. Grossi did not explicitly say that the Syrian reactor’s purpose was to produce a nuclear weapon, but at his press conference in Vienna he emphasized that the then-Syrian officials openly “did not want the world to know what they were doing.”