An explosion followed by an exchange of fire with rebels in a Damascus province on Thursday left at least 21 members of Syria's elite Republican Guards unit dead, a watchdog said.
The Syrian Observatory for Human Rights had reported an earlier toll of 18 from the violence in Qudsaya, west of the capital, but revised it upwards.
"According to a medical source and eyewitnesses, the toll may rise further still," the Britain-based watchdog said.
Observatory director Rami Abdel Rahman said the clashes in Qudsaya were continuing, and that most of the troops who died were killed in the explosion and the remainder in the fighting.
"The rebels likely used a small explosive device (that was) placed near the Republican Guards' living quarters in Qudsaya," said Abdel Rahman.
The army launched a major offensive on Qudsaya and a neighbouring locality on Wednesday, deploying troops heavily there, the Observatory and AFP journalists said.
The killings came two days after the official Al-Baath daily said "the end of security operations throughout Damascus province" was approaching.
The army has waged an intense campaign in recent weeks to try to stamp out the rebellion in Damascus province, pushing the rebels back to their strongholds on the outskirts of the capital.