BEIRUT: Fighting between Syrian regime forces and armed rebels killed four people early Sunday, as a tenuous UN-backed ceasefire entered its second month, monitors and activists said.

The fresh bloodshed and clashes came as the UN mission in Syria has said it now has 176 military observers on the ground, more than half its planned strength of 300.

The observers are tasked with shoring up a ceasefire brokered by UN-Arab League peace envoy Kofi Annan that was supposed to take effect on April 12 but which has been broken daily.

On Sunday a civilian was shot dead by regime forces at a checkpoint in Dmeir, just outside Damascus, while another was killed by a sniper in the capital's northern suburb of Douma, according to the Syrian Observatory for Human Rights.

A rebel commander, Abu Adi, was killed in overnight clashes with regime forces in Douma, while an officer who deserted the army died in a dawn ambush in the eastern province of Deir Ezzor, the Britain-based watchdog said.

The Local Coordination Committee (LCC), a coalition of opposition activists on the ground, said the Syrian army shelled Douma early Sunday and that heavy gunfire was also heard in the suburb.

Clashes also broke out in other parts of the Damascus province, the Observatory reported.

On Saturday, 22 people were killed, including 11 civilians, two deserters and nine regime troops, of which seven were killed during fighting in northwest Idlib province.

Violence continues to rock Syria, where the UN says the 176 military observers are backed by 56 civilian staff to monitor the ceasefire.