A car bomb exploded overnight in the southern Qadam neighbourhood, causing at least one death, said the Syrian Observatory for Human Rights, a Britain-based watchdog that relies on a network of activists and medics on the ground.

In the Mazzeh district, home to many embassies and offices of the security services, three civilians were killed and 12 wounded from mortar fire, it said.

Rebel fighters, meanwhile, clashed with pro-regime Palestinian forces in the southern Hajar al-Aswad district and the nearby Palestinian refugee camp of Yarmuk, which has become a new focal point of violence.

Home to some 148,000 residents, the camp has been the scene of fierce fighting between the rebels and the Popular Front for the Liberation of Palestine-General Command (PFLP-GC), an ally of President Bashar al-Assad.

Wednesday's violence followed bomb attacks in the suburbs of Damascus on Tuesday, including three explosions in the western Qudsaya area, that killed 19 people and wounded more than 50, according to the Observatory.

Elsewhere in the country, regime air power kept up strikes over northern Aleppo and the surrounding region on Wednesday as rebels continued their months-long battle for control of the commercial hub.

The violence on Tuesday left 150 dead, including 79 civilians, 48 soldiers and 23 rebels, said the Observatory, which says 36,000 people have died since the outbreak of the revolt against Assad's regime in March 2011.