(AP) — The U.N.’s human rights office said Tuesday that most of the 108 victims of a massacre in Syria last week were shot at close range, some of them women, children and entire families gunned down in their own homes.

The massacre on Friday in Houla drew new international outrage, with more than half a dozen countries including France and Britain expelling Syrian diplomats in protest in what the White House said was a sign of their “absolute disgust” with Assad’s rule.

Still, spokesman Jay Carney said the White House remains opposed to military action in Syria, reasoning that would only lead to more carnage.

The U.N. peacekeeping chief Herve Ladsous also said there are strong suspicions that pro-government fighters were responsible for some of the killings.

“We are at a tipping point,” special envoy Kofi Annan told reporters Tuesday in Damascus, following a meeting with President Bashar Assad. “The Syrian people do not want the future to be one of bloodshed and division.”

He called on the government and the armed opposition to stop all violence.

The U.N. report indicated that most of the dead were killed execution-style, with fewer than 20 people cut down by regime shelling. The U.N. cited survivors and witnesses blaming the house-to-house killings on pro-government thugs known as shabiha, who often operate as hired muscle for the regime.

“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.”

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, dismembered bodies lying in the streets, then row upon row of the dead laid out before being buried in a mass grave. Some of the videos showed dozens of dead children, some with gaping wounds.

The Syrian regime has denied any role in the massacre, blaming the killings on “armed terrorists” who attacked army positions in the area and slaughtered innocent civilians. But it has provided no evidence to support its narrative nor has it given a death toll.

U.N. investigators have said they found tank and artillery shells in Houla after the attack, but stopped short of blaming regime forces for the killings.

The U.N. said that at least 108 people, including 34 women and 49 children, were killed in the attack that began on Friday and continued through the night on a group of poor farming villages northwest of the central city of Homs.

Speaking to reporters in Geneva, Colville said U.N. monitors who visited the area found that fewer than 20 of the dead were killed by artillery fire. The rest appeared to have been shot at close range.

He said information from U.N. investigators and other sources indicated that many of the victims were killed in the Houla village of Taldaw in two separate incidents. Local residents blamed the killings on pro-regime militias known as shabiha, which sometimes act “in concert” with government forces, he said.

He said a fuller investigation was needed before he could comment on that, and called on Syria to allow free access to U.N. investigators.

The brutality of the killings and the high death toll raised new questions about the ability of a U.N.-brokered plan to end the violence in Syria, which began in March 2011.

According to the state-run news agency, SANA, Assad blamed terrorists and weapons smugglers for scuttling Annan’s peace plan. The regime denies there is any popular will behind the country’s uprising, saying foreign extremists and terrorists are driving the unrest.

The new information provided by the U.N. draws attention to the role of the shabiha in 15 months of violence in Syria. Assad’s government often deploys pro-regime thugs or armed militias to repress protests or carry out more military-style attacks on opposition areas.

They frequently work closely with soldiers and security forces, but the regime never acknowledges their existence, allowing it to deny responsibility for their actions.

A Syrian official denied again on Tuesday any involvement.

“It is irrational that any party who wants to make Annan’s mission a success would ever commit such a massacre,” Deputy Foreign Minister Faisal Mekdad told reporters. He said Syria remained committed to Annan’s plan and “had not committed a single violation.”

Activists have posted videos of tanks and armored vehicles in the middle of cities, a violation of the plan, and U.N. observers said they found spent tank and artillery shells in Houla after the massacre there. Funeral videos also showed local rebels among the mourners – making it unlikely they carried out the killings.

Anti-regime rebels around the country regularly attack military convoys and checkpoints, killing soldiers.

Syria’s international isolation deepened in response to the killings. Governments around the world expelled Syrian ambassadors and diplomats Tuesday, an unusual, coordinated blow to Assad’s regime.

The United States, Britain, Canada, Australia, France, Germany, Italy, Spain and the Netherlands took action Tuesday against Syrian diplomats. Britain’s foreign secretary said the countries involved in Tuesday’s expulsions would also push for tougher sanctions against Syria.

Longtime Syrian ally Russia has largely stood by Damascus, although Moscow is growing increasingly critical – particularly over the Houla massacre. On Tuesday, however, Russian Foreign Minister Sergey Lavrov accused unnamed countries of trying to use the Houla killings “as a pretext for taking military measures.”

He said such nations sought to impede Annan’s plan because it seeks dialogue between Syrians, not regime change.

Syria’s unrest began in March 2011, with protests calling for political change. Government troops swiftly cracked down at the uprising spread, and many in the opposition have taken up arms to defend their towns and attack government troops.

Latest: As U.N. officials cited eyewitness accounts of shadowy gunmen slaughtering whole families in their homes, Western nations expelled Syrian diplomats Tuesday in a coordinated move against President Bashar Assad’s regime over the massacre of more than 100 people.