An explosion followed by an exchange of fire with rebels in Damascus province on Thursday left 18 members of Syria'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 army launched a major offensive on Qudsaya and a neighbouring 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 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 rebellion in Damascus province, especially after armed conflict broke out in the capital in mid-summer.