“We are at a tipping point,” Annan told reporters in Damascus. “The Syrian people do not want the future to be one of bloodshed and division.”

Cranking up the pressure on Assad, the Obama administration gave Syria’s most senior envoy in Washington, the charge d’affaires at the Syrian Embassy, 72 hours to leave the United States. Britain, Canada, Australia, France, Germany, Italy, Spain and Bulgaria also expelled Syrian diplomats.

“We hold the Syrian government responsible for this slaughter of innocent lives,” State Department spokeswoman Victoria Nuland said in Washington. “This massacre is the most unambiguous indictment to date of the Syrian government’s flagrant violations of its U.N. Security Council obligations.”

The massacre in Houla could prove to be a watershed moment in the Syrian crisis, which began in March 2011 with peaceful protests inspired by the wave of uprisings sweeping the Arab world.

Nearly 15 months later, the country is in many ways unrecognizable from the days before the revolt. Assad, once considered a potential reformer in a region filled with aging dictators, is a global pariah. A country that once boasted it was the safest in the Middle East is riven with violence, some of it reminiscent of the worst days of the Iraq war. The economy is in tatters. Syrians are facing price increases for basic goods and endure regular power cuts.

And in some haunting cases, neighbors who have lived side by side for years are turning on each other, driven by sectarian hatred that so many months of violence is laying bare.

According to witnesses, the massacre, which began late Friday in an area about 40 kilometers (25 miles) northwest of the city of Homs, had dangerous sectarian overtones.

The victims lived in the Houla area’s Sunni Muslim villages. But the shabiha forces allegedly behind many of the killings came from an arc of nearby villages populated by Alawites, an offshoot of Shiite Islam.

Most shabiha fighters belong to the Alawite sect, to which the Assad family and the ruling elite also belong. This ensures the gunmen’s loyalty to the regime, built on fears they will be persecuted if the Sunni majority gains the upper hand.

Sunnis make up most of Syria’s 22 million people, as well as the backbone of the opposition. Even as much of the opposition insists the movement is entirely secular, disturbing reports from the ground suggest religious tensions are boiling over.

The volatile sectarian divide makes civil war one of the most dire scenarios.

Activists say as many as 13,000 people have been killed in the uprising. The U.N. put the toll at 9,000 as of March — one year into the revolt — but many hundreds more have died since.

On Tuesday, the U.N.’s human rights office said most of the 108 victims of the Houla massacre were shot at close range. The U.N. report indicated that most of the dead were killed execution-style, with fewer than 20 people cut down by regime shelling.

Deaths from heavy artillery can be blamed on regime forces with relative confidence because rebel fighters do not have such weapons. But it is more difficult to determine who is behind the close-range killings — particularly as Syria sharply restricts media access.

Still, the U.N. cited survivors and witnesses blaming the house-to-house killings on shabiha. Witnesses also told the AP that shabiha were behind the attacks.

“What is very clear is this was an absolutely abominable event that took place in Houla, and at least a substantial part of it was summary executions of civilians, women and children,” said Rupert Colville, spokesman for the U.N. High commissioner for Human Rights.

“At this point, it looks like entire families were shot in their houses,” he said.

It is not clear what touched off the convulsion of violence. Houla activists reached by Skype said government troops shelled the area after anti-government protests on Friday and clashed with local rebels. Later, shabiha from nearby villages swept through the area, stabbing residents and shooting them at close range.

Videos posted online by anti-regime activists show explosions in Houla and dismembered bodies in the streets, then row upon row of the dead laid out before being buried in a mass grave. Some videos showed dozens of dead children, some with gaping wounds.

According to the state-run news agency, SANA, Assad on Tuesday blamed terrorists and weapons smugglers for scuttling the peace plan, which called for a cease-fire and dialogue with the opposition. The regime denies there is any popular will behind the country’s uprising, saying foreign extremists and terrorists are driving the unrest.

Although Damascus has remained largely impervious to international condemnation over the course of the uprising, Tuesday’s diplomatic squeeze will increase pressure on Syria’s remaining allies, including Russia.

Russia has provided a key layer of protection for the Syrian government in the uprising. Russia and China have used their veto power to block U.N. resolutions against Assad. But Russia has grown increasingly critical of Damascus in recent months, and the Houla massacre has prompted some of the strongest condemnations yet from Moscow.

Russian President Vladimir Putin is traveling to Germany and France this week and is likely to come under even greater criticism for his support of the regime.

“We have to continue our work with the Russians,” British Foreign Secretary William Hague said. “We will continue to discuss this with Russia. Russia has particular leverage on the regime and therefore has a particular role in this crisis.”

Despite some shift in Russia’s stance recently, Russian Foreign Minister Sergey Lavrov said Tuesday the Houla massacre must not be a pretext to push for military intervention from outside. Instead, he urged all sides to focus on the Annan plan.

Hague said that the situation in Syria is more complicated than what international powers faced in Libya last year, when a U.N. resolution ushered in NATO military intervention against dictator Moammar Gadhafi’s regime.

White House spokesman Jay Carney said the Obama administration remains opposed to military action, reasoning that it would lead only to more carnage. He said the U.S. will continue offering non-lethal assistance to the Syrian people and said Tuesday’s coordinated move to expel Syrian diplomats was a signal of the international community’s “absolute disgust” with Assad’s rule.

Assad still commands a strong army that has proven largely unwilling to turn on him. The entire structure of the state has been built to preserve Assad’s power, with the military, the police and security services — even the economy — tied up with the survival of his presidency.

But as the violence engulfs the country, many see Assad’s departure as the only way out.

Fawaz Zakri, a member of the opposition Syrian National Council, urged action by the U.N. Security Council, saying the world body “must do something to save the Syrian people’s souls.”

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Associated Press writers Zeina Karam and Ben Hubbard in Beirut, Angela Charlton in Paris, Frank Jordans in Geneva, Edith M. Lederer at the United Nations, Albert Aji in Damascus, Syria, and Selcan Hacaoglu in Ankara, Turkey, contributed reporting.