An explosion followed by an exchange of fire with FSA rebels in Damascus province on Thursday left 25 members of Basha al-Assad's elite Republican Guard force dead, a watchdog said.
"Most of them died in the explosion", said Syrian Observatory for Human Rights director Rami Abdel Rahman, who added that the clashes in Qudsaya, west of the capital, were continuing.
The Alawite army launched a major offensive on Qudsaya and a neighboring locality on Wednesday, deploying troops heavily there, the Observatory and AFP journalists said.
"The rebels likely used a small explosive device, but it was placed near the Republican Guards' living quarters in Qudsaya", said Abdel Rahman.
Some of the dead fell during clashes with the rebel Free Syrian Army, he said.
The killings came two days after the Alawite Al-Baath daily said "the end of security operations throughout Damascus province" was approaching.
The Alawite army has waged an intense campaign in recent weeks to try to stamp out rebellion in Damascus province, especially after armed conflict broke out in the capital in mid-summer.