WASHINGTON — The U.S. and 10 other countries expelled Syrian diplomats Tuesday after blaming government forces for last week’s massacre of more than 100 villagers in central Syria but again stopped short of calling for more aggressive action to end the violence.

Despite expressions of outrage and agreement that the situation in Syria has reached what U.N. special envoy Kofi Annan called a “tipping point,” the Obama administration and its major allies said they would continue to rely on diplomatic, political and economic pressure against Syrian President Bashar Assad.

U.N. monitors who visited the village of Houla, outside Homs, found that all but about 20 of 108 victims had been “summarily executed,” including “entire families … shot in their houses,” Rupert Colville, spokesman for the U.N. High Commissioner for Human Rights told reporters in Geneva. The rest, he said, were killed by artillery fire. Nearly half of the victims were younger than 10.

Annan, who traveled to Damascus, the Syrian capital, in a bid to salvage a six-point U.N. peace plan, said he had appealed to Assad for “bold steps – not tomorrow, now – to create momentum for implementation of the plan.” He called for an immediate end to violence by the Syrian military and government-backed militias and urged armed rebels to stop attacks.

Others used sharper language to assign blame for one of the worst atrocities since the anti-Assad Syrian uprising began 14 months ago.

“These were regime-sponsored thugs who went into villages, went into homes and killed children at point-blank range and their parents,” State Department spokeswoman Victoria Nuland said. She said that the Syrian “thug force mirrors the same force that the Iranians used” to put down a nascent political uprising in 2009.

Herve Ladsous, the U.N. undersecretary general in charge of peacekeeping, said the government was responsible for those killed by shelling. The rest, he said, were shot or stabbed in attacks that “probably points the way to the shabiha.” The term refers to pro-government militias belonging to Assad’s Alawite sect; Houla residents said the shabiha had gone house to house on a killing spree in the Sunni village Friday night.

As more details of the massacre emerged, 11 governments – including the U.S. and its major NATO allies – made coordinated announcements that they were expelling top Syrian diplomats. The Obama administration said it had given Syrian charge d’affaires Zuheir Jabbour and several other senior diplomats 72 hours to leave, although the embassy in Washington would stay open with low-level staff.

But despite the harsh denunciations the international community is too divided to come up with effective ways of putting pressure on the Assad regime, said Salman Shaikh, director of the Brookings Doha Center. Russia, with veto power in the U.N. Security Council, remains opposed to taking tougher action against its chief Middle Eastern ally, as does China.