Damascus: Syrian regime forces shelled rebel areas and clashed with gunmen on Sunday in a new test to a shaky UN-backed ceasefire as international monitors prepared to arrive in the unrest-hit country.

Forces loyal to President Bashar al-Assad subjected the Khaldiyeh and Bayada neighbourhoods of the flashpoint central city of Homs to their fiercest bombardment since the truce went into force at dawn on Thursday, monitors said.

"The bombardment of Khaldiyeh intensified this morning with an average of three shells a minute," the head of the Syrian Observatory for Human Rights, Rami Abdel Rahman, told AFP.

Three civilians died in shelling of the city on Saturday, among 14 people killed nationwide ahead of a UN Security Council vote approving the dispatch of the observer mission to monitor the truce.

Elsewhere, rebel fighters clashed with security forces in the town of Al-Bab, in the northern province of Aleppo, close to the town's State Security police headquarters, the Observatory said.

A police station in the town also came under fire, the Britain-based watchdog added.

Thirty-two people have been killed since the ceasefire brokered by UN-Arab League peace envoy Kofi Annan took effect, most of them civilians, the Observatory said.

The death toll is sharply down on pre-ceasefire levels after Syria announced it was halting military operations against the rebels on Thursday.

But regime forces have so far ignored another key element of Annan's peace plan -- an undertaking to withdraw tanks from towns and cities, the Observatory said.

"Since the Annan plan took effect, there has not been any change in the level of military deployment. Roadblocks and tanks remain," Abdel Rahman told AFP.

The first half-dozen observers from the advance team for the UN mission were due in Damascus later on Sunday.

Annan wants the mission's numbers to swell to more than 250 but they can only leave if the fragile truce holds, and need another UN resolution to approve their deployment.

The first group boarded a plane from New York straight after the Security Council resolution was passed.