Beirut - 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."
U.S. State Department spokeswoman described an "absolutely indefensible, vile, despicable massacre against innocent children, women, shot at point blank range by regime thugs."
U.N. 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.
Western countries that have called for Assad to step down hope the Houla killings will tip global opinion, notably that of Syria's main protector Russia, towards more effective action.
The killings have contributed to doubt about whether a peace plan backed by Annan, a former U.N. 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.
"We are at a tipping point"
Annan said that in his talks with the Syrian leader in Damascus he "conveyed in frank terms the grave concern of the international community about the violence in Syria, including the recent shocking events in Houla."
"We are at a tipping point. The Syrian people do not want the future to be one of bloodshed and division. Yet the killings continue and the abuses are still with us today."
More than 10,000 people have died in the uprising against Assad and a crackdown by his loyalists, which began in March last year as part of a wave of revolts across the Arab world.
Western leaders have loudly called for Assad to go for months, but have so far shown little appetite for the sort of armed intervention that toppled Libya's Muammar Gaddafi last year. Any suggestion that force might be threatened has been blocked at the U.N. Security Council by Moscow and Beijing.
French President Francois Hollande said armed intervention could not be ruled out, but only with Security Council support.
"Military intervention is not excluded, provided it is carried out under the auspices of international law, namely a Security Council resolution," he told France 2 TV. "It is down to myself and others to convince Russia and China, and also to find a solution which is not necessarily a military one."
White House spokesman Jay Carney said the United States had not taken any options - including military intervention - off the table, but believed armed action was not the right course at present because it would lead to greater carnage and chaos.
Moscow backed a non-binding U.N. Security Council text on Sunday criticising the use of artillery and tanks in Houla, but has twice vetoed tougher resolutions. It has shown little sign of changing its stance this week.
Vladimir Chizhov, Moscow's envoy to the EU, criticised the Western expulsions of Syrian envoys: "I believe it rarely is conducive to maintaining dialogue with the other side."
Foreign Minister Sergei Lavrov said Moscow was "alarmed that some countries ... are starting to use this event as an excuse to put forth demands of the need for military action in an attempt to put pressure on the U.N. Security Council."
"We are troubled by the ceaseless attempts to frustrate Kofi Annan's peace plan."
Annan urged the armed opposition to cease violence but appealed first to the government, as the stronger party, to take "bold steps now - not tomorrow, now" by stopping all military operations and showing "maximum restraint".
The plan calls for the government to withdraw all heavy weapons from towns and cities, followed by a cessation of fighting and dialogue with the opposition, but has stalled at the first hurdle.
Assad's government denied having anything to do with the deaths in Houla, or even having heavy weapons in the area, despite the contrary evidence found by United Nations monitors.
Assad himself repeated to Annan Syria's line that "terrorist groups" - Syria's term for the rebels - were stepping up killings and kidnappings across the country.
Syrian Deputy Foreign Minister Faisal Mekdad told reporters: "Syria has not committed a single violation of Annan's plan or the initial understanding between Syria and the United Nations.
"At the same time, the other party has not committed to a single point. This means that there is a decision by the armed groups and the opposition not to implement Annan's plan and to make it fail."
The estimated number of internally displaced Syrians has more than doubled to 500,000 since an April 12 ceasefire and the flow of refugees abroad has gathered pace again, U.N. officials said on Tuesday.
Spent shells and tank tracks
U.N. monitors found spent shells and fresh tank tracks in Houla, evidence of weaponry that Syria's lightly-armed rebels do not have in their arsenal.
But the U.N. human rights office in Geneva said the bulk of the 108 mostly civilian dead in Houla had been executed at close range. Survivors told U.N. investigators that the killers were pro-Assad "shabbiha" militiamen, who in the past have assaulted and intimidated hotbeds of opposition to Assad.
"What is very clear is that this was an absolutely abominable event that took place in Houla, and at least a substantial part of it were summary executions of civilians - women and children," said Rupert Colville, spokesman for the U.N. human rights office in Geneva.
He said 49 children and 32 women were among the dead. "At this point, it looks like entire families were shot in their houses."
The report was at odds with a note sent by Syria to the Security Council that said: "Not a single tank entered the region and the Syrian army was in a state of self-defence ...
"The terrorist armed groups ... entered with the purpose of killing and the best proof of that is the killing by knives, which is the signature of terrorist groups who massacre according to the Islamist way."
Gruesome video footage distributed by opposition activists has helped to shake world opinion out of growing indifference to a conflict in which more than 10,000 have been killed.
Opposition sources said rebels had killed 20 soldiers in heavy fighting close to the border with Turkey.
They said six civilians and six rebels, including two commanders, had also been killed over the past 24 hours in fighting that began when the army launched an offensive with tanks and helicopters to retake the region around Atareb.