Israeli air raids on three military sites near Damascus killed at least 42 soldiers at the weekend, the Syrian Observatory for Human Rights said in an updated toll on Monday.
"At least 42 soldiers were killed in the strikes, and another 100 who would usually be at the targeted sites remain unaccounted for," Observatory director Rami Abdel Rahman told AFP.
The Britain-based monitoring group had earlier given a toll of at least 15 soldiers killed.
Syria says Israel targeted three military sites near Damascus early Sunday, with a diplomatic source in Beirut saying the raids targeted a military facility, weapons depot and anti-aircraft unit.
It has yet to give an official toll, but the foreign ministry said in a letter to the United Nations that the Israeli "aggression" caused deaths, 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 strikes targeted weapons destined for Lebanese group Hezbollah, which is allied with the regime of Syrian President Bashar al-Assad.
Elsewhere in the country on Monday, the Observatory said Syrian military planes bombed the districts of Qabun and Jubar in the capital.
They also targeted the rebel-held town of Qusayr in central Syria, which regime troops are trying to recapture with support from Hezbollah fighters.
In the north, fighting raged around Abu Dhrur airport in Idlib province and Minnigh airport in Aleppo province, where rebels have reportedly seized large swathes of territory.
The army took control of a string of villages in the south of Aleppo province, and eight soldiers, including two officers, were killed when rebels shot down a helicopter in the eastern province of Deir Ezzor, the watchdog added.
Violence throughout Syria on Sunday killed at least 137 people, according to the organisation, which relies on a network of activists, doctors and lawyers on the ground.
The dead included 47 rebels, 49 civilians and 41 soldiers, including the 15 killed in the Israeli attacks.