The United States and several other Western nations moved Tuesday to expel Syrian diplomats amid mounting international outrage over the massacre Friday of more than 100 villagers in central Syria, most of them women and children.

The moves came as the U.N. special envoy for Syria, Kofi Annan, warned President Bashar al-Assad that the crisis in Syria had reached “a tipping point” following the killings overnight Friday of 108 civilians in the village of Houla outside Homs.

Annan, a former U.N. secretary general, told reporters after his talks with Assad in the Syrian capital, Damascus: “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.” He said he appealed to Assad to act immediately to halt violence by his forces and government-backed militias, and he urged armed rebels to stop attacks as well.

According to the United Nations, nearly half of the civilians killed in Houla were children under age 10. The dead also included 34 women, and most of the victims apparently were shot at close range, the U.N. human rights office said.

The U.S. government is giving Syria’s charge d’affaires, the country’s top diplomat currently in Washington, 72 hours to leave the United States, administration officials said. Earlier, Britain, France, Germany, Spain, Italy, the Netherlands and Australia announced that they were expelling Syrian ambassadors or other top envoys to protest the killings. Canada announced that all Syrian diplomats were being ejected from the country. Bulgaria and Switzerland were among other countries reported to be following suit.

French Foreign Minister Laurent Fabius described Assad in a newspaper interview as “the murderer of his people” and said he “must relinquish power.” Australian Foreign Minister Bob Carr vowed to pursue “a unified international response to hold those responsible to account.”

Annan went to Damascus in a bid to salvage a six-point peace plan endorsed last month by a U.N. Security Council resolution but now clearly in jeopardy from the rising violence. The Local Coordination Committees, an opposition group, said 55 people were killed Tuesday.

“I appealed to him for bold steps now — not tomorrow, now — to create momentum for the implementation of the plan,” Annan said at a news conference.

He also urged “the armed opposition to cease acts of violence.” But he made it clear he held the government — “as the stronger partner in this conflict” — primarily responsible for the plan’s failure to halt the bloodshed.

“This means that the government, and all government-backed militias, could stop all military operations and show maximum restraint,” he said. “I appealed to the president ... to be bold for the Syrian people.”

He also said he warned Assad: “the international community will soon be reviewing the situation.”

The Houla massacre was one of the worst single incidents of the 14-month old uprising against Assad’s rule and has heightened concerns that Syria is sliding into an unstoppable conflict that could spread beyond its borders. More than 10,000 people are believed to have died in the uprising.

After initially blaming Syrian government artillery for most of Houla’s 108 deaths, including those of 49 children, the United Nations said Tuesday it now believes that pro-government militias were largely responsible.

Rupert Colville, a spokesman for the U.N. High Commissioner for Human Rights, told journalists in Geneva that U.N. monitors had found that fewer than 20 of the 108 victims had died in artillery fire, the Associated Press reported.

“Most of the rest of the victims were summarily executed in two separate incidents,” Colville said. “At this point it looks like entire families were shot in their houses.”

Witnesses blamed pro-government militias known as shabiha for the attacks, he said.

Houla residents have offered a similar account, saying that most of the killings were carried out by armed civilians belonging to Assad’s minority Alawite sect, who went house-to-house in the Sunni village killing entire families overnight on Friday.

The Syrian government has denied responsibility and accused the opposition of carrying out the killings.

Deputy Foreign Minister Faisal al-Miqdad rejected charges that the Syrian government is primarily responsible for the failure of the peace plan.

“During this time, Syria has not committed a single violation of the plan,” Miqdad told reporters Tuesday.

In Washington, Gen. Martin Dempsey, chairman of the Joint Chiefs of Staff, warned that such “atrocities” may end up triggering international military intervention.

Speaking on Fox News, he stressed, however, that he would like to see the international community exert greater diplomatic pressure on Assad before exploring military options.

“You’ll always find military leaders to be somewhat cautious about the use of force, because we’re never entirely sure what comes out on the other side,” he said. “But, that said, it may come to a point with Syria because of the atrocities.”

Activists reported fresh violence on Monday, claiming that at least 34 people died in shelling by government forces overnight Sunday in the city of Hama.

Annan arrived in Damascus on Monday in an effort to salvage the peace plan, which has increasingly been called into question by the escalating violence. He said in a statement that he was “personally shocked and horrified” by the deaths in Houla, which the U.N. Security Council blamed on the Syrian government in a sharply worded statement issued Sunday.

“This was an appalling crime, and the Security Council has rightly condemned it,” Annan said in a statement issued on his arrival in Damascus.

Sunday’s Security Council statement blamed most of the deaths in Houla directly on “government artillery and tank shellings on a residential neighborhood” in violation of a Security Council resolution that calls on Syria to withdraw troops and tanks from residential areas.

The statement was the international body’s strongest condemnation yet of the Syrian government’s behavior, and it signaled the harshest criticism of Damascus to date by Russia and China, Syria’s allies.

But it contained no threat of new measures against the Syrian government, and the international community is too divided to come up with effective ways of putting pressure on the regime, said Salman Shaikh, director of the Brookings Doha Center. Notably, he said, Russia remains opposed to taking tougher action against its chief Mideast ally.

“This gruesome incident could give real momentum to diplomatic efforts, but so far it’s not had much effect,” Shaikh said. “There is still the hope among key Western powers that the Russians will come round. But I don’t think it’s really changing tack.”

The differences were apparent Monday during a visit by British Foreign Secretary William Hague to Moscow, where he and his Russian counterpart, Sergei Lavrov, asserted their support for Annan’s peace plan. Lavrov made it clear that Russia also holds armed rebels responsible for much of the violence that appears to have escalated in recent days.

“The guilt has to be determined objectively,” Lavrov told reporters. “No one is saying that the government is not guilty, and no one is saying that the armed militants are not guilty.”

Hague said the rebels also are at fault but stressed that he believes the government “has the primary responsibility” for the violence.

Staff writer Karen DeYoung in Washington contributed to this report.