Western powers expelled Syria's envoys on Tuesday in outrage at a massacre of 108 people, almost half of them children, and peace envoy Kofi Annan urged President Bashar al-Assad to halt the bloodshed as "a tipping point" had been reached.

The killings in the town of Houla drew a chorus of condemnation from around the world, with the United Nations saying entire families were killed in their homes on Friday, some by army tanks and others probably by pro-Assad militia.

"Bashar al-Assad is the murderer of his people," French Foreign Minister Laurent Fabius told Le Monde. "He must relinquish power. The sooner the better."

US State Department spokeswoman described an "absolutely indefensible, vile, despicable massacre against innocent children, women, shot at point blank range by regime thugs."

UN peacekeeping chief Herve Ladsous, whose monitors are in Syria, contradicted the Assad's government assertion that the killings were carried out by terrorist gangs.

"Part of the victims had been killed by artillery shells, now that points ever so clearly to the responsibility of the government. Only the government has heavy weapons, has tanks, has howitzers," Ladsous told reporters, adding:

 "But there are also victims from individual weapons, victims from knife wounds and that of course is less clear but probably points the way to the (pro-Assad) shabbihas, the local militia."

The United States, France, Britain, Canada, Germany, Italy, Spain, Australia and Bulgaria gave Syria's envoys hours or days to leave their capitals in a coordinated move meant to isolate Assad further diplomatically.

Some had already expelled ambassadors or downgraded ties and so, like Washington, ordered out less senior charges d'affaires.

The killings have contributed to doubt about whether a peace plan backed by Annan, a former UN secretary general, has any chance of success. Some of Assad's opponents say Annan's plan is only making the situation worse by buying Assad time.