At least 22 people were killed on Thursday in violence across Syria, including 10 in the central province of Homs, and as car bombs exploded in Damascus and the northwest city of Idlib, monitors reported.

A suicide bomber detonated an explosives-packed vehicle in a suburb of the capital, wounding 14 people and damaging one of Shiite Islam's holiest shrines, state media and witnesses reported.

State news agency SANA said the vehicle exploded in a garage 50 metres (yards) from the Sayyida Zeinab shrine. There was "substantial damage in the area of the blast," and "the terrorist who carried out the operation was killed."

The Observatory reported widespread raids by troops later in the capital.

A car bomb in Idlib city targeted a military checkpoint, the Syrian Observatory for Human Rights said, adding that a number of soldiers were killed or wounded.

Eight people, including three opposition fighters, were killed during clashes between troops and rebels in the central city of Homs, it said, while also reporting renewed regime shelling of Rastan in the province of the same name and of Daraa in southern Syria.

Two opposition fighters, including Ahmed Bahbouh -- head of the rebel military office in Rastan and a leading dissident -- were killed at the entrances of the rebel-held town, which the regime has been trying to overrun for months.

Troops bombarded Rastan "using helicopters and mortars, killing and wounding a large number of rebel fighters," the watchdog reported.

The Observatory accused the UN observers of "silence," saying that they "do not move until after a city is defeated by regime troops, as happened in Al-Haffe."

That was a reference to a town in Latakia province that was bombarded for eight days before troops moved in.

In the city of Daraa, five people were killed before dawn in the neighbourhood of Tareek al-Sad, which was heavily shelled, the Britain-based Observatory said.

"Government forces have surrounded the neighbourhood of Tareek al-Sad in preparation to storm the area," it said.

Six people were killed in the central province of Hama, including three soldiers, two officers and a girl, while another civilian was killed in Damascus province, according to the Observatory.

At least 77 people were killed across Syria on Wednesday, including 49 civilians, 21 soldiers and seven rebels, the watchdog said.

More than 14,400 people have been killed in the 15-month revolt in Syria, the majority of them civilians, according to the Observatory.

The escalation in violence follows an assessment by UN peacekeeping chief Herve Ladsous on Tuesday that Syria was now locked in a civil war, with regime forces having lost control of "large chunks of territory."