Victims of the Houla massacre included dozens of children
Western states have announced they are expelling senior Syrian diplomats following the killing of 108 people in the Houla region of Syria on Friday.
Most of the victims in Houla were summarily executed, the UN says.
UN-Arab League envoy Kofi Annan said, after talks with President Bashar al-Assad, Syria was at a “tipping point”.
He said he had asked Mr Assad to take “bold steps” to see that his peace plan was implemented.
President Assad blamed the violence on “terrorists”. His remarks were quoted by state TV.
Earlier, UN human rights spokesman Rupert Colville said initial investigations had suggested that most of those killed in the village of Taldou, near Houla, were summarily executed.
He said 49 children and 34 women were among the victims. UN observers who visited Taldou said many of the victims had been killed by close-range gunfire or knife attacks.
Eyewitnesses told the BBC that pro-government shabiha militiamen had carried out the killings. Survivors said they had hidden or played dead.
Syrian leaders insist that the massacre was the work of “terrorists”, aiming to derail the peace process and provoke intervention by Western powers.
Violence continued on Tuesday, with nearly 50 people killed in various incidents, according to activists.
‘Heinous and brutal’
US state department spokesperson Victoria Nuland announced that the Syrian charge d’affaires in Washington had been given 72 hours to leave the country.
“We hold the Syrian government responsible for this slaughter of innocent lives [in Houla],” she said.
Speaking later, White House spokesman Jay Carney said the Obama administration remained opposed to military action in Syria, on the grounds that it might only lead to more carnage.
However, French President Francois Hollande, speaking in Paris, said an “armed intervention” was not being ruled out, provided it followed deliberation by the UN Security Council.
He said he would discuss Syria with his Russian counterpart, Vladimir Putin, when visits Paris on Friday.
In other reaction
Canada denounced the Syrian government’s “heinous and murderous acts”
Australia described the Houla massacre as a “hideous and brutal crime”
Spain talked of “unacceptable repression”
The Dutch declared Syria’s ambassador to the Netherlands, who is also ambassador to Belgium and lives in Brussels, as “persona non grata”
Syria’s ambassador to Switzerland was declared “persona non grata”
Bulgaria said it was expelling Syria’s interim ambassador and two other diplomats
Syria’s charge d’affaires in London has been given seven days to leave.
“The international community is appalled by the violence that has continued, by the behaviour of the regime, by the murder of so many innocent people,” British Foreign Secretary William Hague said.
News of the expulsions was welcomed by Ronya Kaysar, a representative of Syrian opposition groups based in Qatar.
It would put “great pressure on the Syrian regime”, she told BBC News.
Despite the international show of protest, it is unclear whether the mass diplomatic expulsions will change much on the ground, Bridget Kendall, the BBC’s diplomatic correspondent, says.
President Assad said the success of Mr Annan’s peace plan depended on halting what he called terrorist actions and stopping arms-smuggling.
Speaking to reporters, Mr Annan said the six-point international peace plan for Syria was not being implemented “as it must be”.
“I appealed to him [Mr Assad] for bold steps now – not tomorrow, now – to create momentum for the implementation of the plan,” he said.
Calling on the Syrian government and all government-backed militias to show “maximum restraint”, he also asked the “armed opposition to cease acts of violence”.
Russia, which supplies arms to the Syrian government and has blocked UN resolutions calling for action against Damascus, has blamed both sides for Friday’s massacre.
Its Foreign Minister, Sergei Lavrov, expressed concern that “certain countries” were beginning to use the Houla massacre “as a pretext for voicing demands relating to the need for military measures to be taken”.
Speaking to Mr Annan by telephone on Tuesday, he again urged an end to violence on all sides, and called for an impartial investigation into the killings in Houla under the aegis of the UN observer mission.
A meeting of the so-called Friends of Syria group is to meet in France in July, President Francois Hollande’s office said.
Mr Annan asked President Assad to take bold actions today, not tomorrow, to end the violence. He said the situation had reached a tipping point. He believes that his plan has not been implemented. He urged all parties to implement it comprehensively.
The Syrian government denied it had breached the agreement and blamed the violence on terrorist groups – a typical message from a defiant government. But many here believe the massacre in Houla is a turning point.
It has pushed shopkeepers in Damascus who have not taken any public position over the past 15 months to go on strike. And it has encouraged more people to protest, even in the streets of central Damascus. They were violently dispersed by the security forces.
Many here in Syria attach great hope to the UN envoy’s efforts but there are a lot of doubts about the government’s willingness to make concessions. The fear is if no real action is taken now, violence could reach the centre of the capital, and the country will slip into a civil war.
Kofi Annan: “The Syrian people do not want their future to be one of bloodshed and division”
Taldou, Houla region
The region of Houla, in the west of Syria, comprises several villages and small towns
The village of Taldou lies around 2km south-west of the main town, also called Houla
The area is in the province of Homs, which has seen heavy fighting in recent months
Houla’s villages are predominantly Sunni Muslim, but the region is ringed by a number of Alawite villages – the sect of the Syrian president Bashar al-Assad
Houla: How a massacre unfolded
Press split on Syria mass killing
Timeline: Syria’s massacres