Syrian forces have unleashed a barrage of artillery on the battered city of Homs for hours before dawn, killing more than 200 people in what appeared to be the bloodiest episode in the 11-month-old uprising, activists said.

The government denied the assault, saying the reports were part of a "hysterical campaign" of incitement by armed groups against Syria, meant to be exploited at the UN Security Council as it prepares to vote on a draft resolution backing an Arab call for President Bashar Assad to give up power.

With Syria growing increasingly chaotic, Western and Arab countries have stepped up their push for a UN resolution to pressure Assad. A vote was scheduled for today, but negotiations are continuing to the last minute as Russia, a strong ally of Syria, signalled it would veto any call for Assad's removal.

In a blunt warning to Washington, Russian foreign minister Sergey Lavrov said that if a resolution is put to a vote without taking Russia's opinion into account it will only lead to "another scandal" at the Security Council.

French foreign minister Alain Juppe countered that after the Homs bloodshed - which he called a "crime against humanity" - "those who block the adoption of such a resolution are taking a grave historical responsibility".

Tunisia expelled Syria's ambassador in response to the "bloody massacre" in Homs. Angry Syrians stormed their embassies in Berlin, London, Athens, Cairo and Kuwait city, clashing with guards and police. Protesters in Cairo set fire to part of the embassy building.

Hours after the Homs assault eased, security forces in the Damascus suburb of Daraya opened fire on a funeral procession for victims of a shooting a day earlier, killing seven people, activists said.

There were signs that the bombardment in Homs, Syria's third largest city, was in response to moves by army defectors to solidify their control in several neighbourhoods. There were reports that defectors set up new checkpoints in several areas, and two activists from Homs said defectors attacked a military checkpoint in the Khaldiyeh district on Thursday night and captured 17 soldiers.

If defector activity was the spark, the assault signals a new willingness by the regime to unleash more devastating force against the dissidents. The defectors, part of a force called the Free Syrian Army, have grown increasingly bold in attacks on the military and attempts to take overt control in pro-opposition areas.

Khaldiyeh, a mainly Sunni neighbourhood in the mixed city, took the brunt of the assault. Residents described a night of ceaseless shelling that sent them fleeing to lower floors and basements of buildings.