Israeli strikes on Syrian military targets at the weekend killed at least 15 soldiers and dozens more were unaccounted for, the Syrian Observatory for Human Rights watchdog said on Monday.
"At least 15 soldiers were killed, and dozens more are missing" after the strikes near Damascus early on Sunday, said the Observatory's Rami Abdel Rahman.
"These three sites (targeted) would usually have around 150 soldiers in them, but it's not clear if they were all there at the time of the strikes."
Syria said on Sunday that Israel had targeted three military sites near Damascus, with a diplomatic source in Beirut saying the attacks were against a military facility, a weapons depot and an anti-aircraft unit.
The Syrian government has not given an official toll from the attacks, but the foreign ministry said in a letter to the UN that the Israeli "aggression caused deaths and injuries and serious destruction."
The attacks, which residents said sounded like an earthquake and lit up the night sky, were the second time in a week that Israel has reportedly targeted Syrian sites.
An senior Israeli source said the target was weapons destined for Lebanese group Hezbollah, which is allied with the regime of Syrian President Bashar al-Assad.