The brutal killings of dozens of women and children in the central Syrian city of Homs prompted the council representing Syria’s opposition to call Monday for “urgent” international military intervention to protect civilians and prevent civil war as diplomatic efforts to resolve the country’s escalating conflict faltered.

The Syrian National Council — a deeply divided umbrella group whose efforts to unite the opposition have failed in part because of differences over military intervention — issued the call after videos showing the mutilated corpses of at least 45 victims were posted overnight on the Internet.

Activists and medical workers said the videos offered evidence of what they called a massacre in Homs by pro-government militias, which embarked on a murderous rampage Sunday evening in the Sunni neighborhood of Karm el-Zeitoun.

Groups of people known as shabiha, or armed thugs, raided the area from an adjoining neighborhood populated by members of President Bashar al-Assad’s minority Alawite sect and began randomly shooting, stabbing and burning civilians, the activists said.

The Syrian government countered with allegations that “terrorist armed groups” had killed the victims and then filmed their bodies, aiming to influence a debate on Syria at the U.N. Security Council on Monday and to shore up opposition calls for “foreign interference in Syria.”

With foreign journalists barred from independent reporting in Syria, the claims were impossible to verify. But the footage — showing mangled, bloodied corpses of women and children piled in the living room of a home that serves as a field hospital — signaled a new low in the violence engulfing the country as the government steps up its efforts to crush a nearly year-long revolt against Assad’s rule and as the opposition increasingly fights back.

The indications that the killings may have had a sectarian motive also underscored the danger that the conflict is rapidly sliding toward a vicious and most likely sectarian civil war that risks spreading beyond Syria’s borders.

The killings came only hours after the United Nations’ new Syria envoy, Kofi Annan, concluded a fruitless mission aimed at persuading the government in Damascus to accept a cease-fire and begin negotiations with the opposition.

Pressure on Russia, China

At the United Nations, Secretary of State Hillary Rodham Clinton and her European and Arab colleagues tried to press Russia and China, which vetoed a recent resolution calling on Assad to relinquish some powers, to support a tougher line against the regime.

Clinton sounded upbeat Monday about the prospects of Russian support for a U.S.-crafted draft resolution condemning Syria and backing Annan’s mediation efforts.

“I had a constructive conversation with Russian Foreign Minister [Sergei] Lavrov,” she told reporters. “I think he has heard clearly how strong the feelings are in the region and on the Security Council, and that we expect all nations, including Russia and China, to join us now in pressing the Assad regime to silence its guns.”

At a news conference in Istanbul, leaders of the Syrian National Council cited the failure of Annan’s visit to extract concessions from the Syrian government, as well as the Homs killings, for their decision to support the calls for international military intervention that have long been made by activists on the ground.

“This massacre, after Annan's visit, is proof that the time for peaceful solutions is over,” Syrian National Council official George Sabra told reporters.

A statement issued by the council said the intervention should include the imposition of a no-fly zone; the establishment of safe corridors and a buffer zone where civilians could seek respite from the killing; and “an organized and speedy operation to arm the Free Syrian Army,” the ad hoc rebel movement that has sprung up in various locations to fight the government.

These are demands that had long been embraced by many protesters and the Free Syrian Army, but that the council, comprising mostly exiled opposition leaders, had resisted. Western nations, including the United States, have repeatedly cited divisions within the Syrian opposition on the issue when explaining their own reluctance to take military action.

“I think the divisions are over,” said Radwan Ziadeh, a member of the council’s foreign affairs committee, who lives in Washington. “Now it is clear that the Assad regime has left us with no other option. To see women and children slaughtered shows that this is systematic, and international intervention is the only option to prevent civil war.”

The latest killings underscored fears surrounding the fate of civilians in Homs as the Syrian government presses its offensive to regain control of the city, suppress dissent and crush the Free Syrian Army.

Speaking at the airport in Ankara, Turkey, Annan said he had heard “grave and appalling reports of atrocities and abuses,” adding that “the killing of civilians must stop now.”

Infants injured

A medical worker in Homs, who spoke on the condition of anonymity because he expects to be killed if it becomes known that he talks to journalists, said the killings apparently occurred late Sunday, hours after Annan left Damascus, when shabiha militias stormed into Karm el-Zeitoun from the nearby Alawite neighborhood of Wadi Dahab.

In addition to about 50 reported deaths, the hospital where he works had received 57 injured patients, 30 of them children, including several infants as young as 4 months old. They had been stabbed and shot, and some had been burned, he said. Those he spoke to told him that some of the women had been raped and that among the dead were eight members of a Palestinian family.

Omar Shakir, a Syrian activist in Lebanon who fled the recent onslaught against the Baba Amr neighborhood but continues to monitor developments in Homs, said 46 women and children were killed, along with a handful of men. Activists discovered the bodies and carried them to the neighborhood of Bab al-Sibaa, a nearby opposition stronghold, after a resident of Karm el-Zeitoun fled and raised the alarm.

In the early hours of Sunday morning, scenes of the bodies piled up in the home used as a field hospital were live-streamed over the Internet. They showed the disfigured corpses wrapped in bloodied blankets.

The account given by the official Syrian Arab News Agency said, however, that the bodies belonged to victims of earlier kidnappings by terrorists. It quoted several men as saying they recognized their relatives from the videos.

Another video posted late Sunday showed about a dozen men who had been shot dead in the Jobar district of Homs, where many of those who fled the assault against Baba Amr, a former opposition stronghold, had taken refuge.

Since Syrian troops overran Baba Amr this month, after reportedly killing hundreds in a nearly four-week siege, they have intensified efforts to crush resistance in other parts of Homs that had fallen out of government control. Communications to most parts of the city have been severed, and many activists are on the run, limiting the flow of information from Homs and making it even harder than usual to establish what is happening there.

Russia reached agreement over the weekend on a five-point plan that would call for an immediate end to the violence in Syria, demand unimpeded access for humanitarian aid workers and support Annan’s mediation efforts. It would also prohibit any foreign intervention in Syria and establish impartial monitoring. The Russian-Arab agreement did not include a reference to a previous Arab League demand that Assad relinquish powers to a vice president to oversee negotiations with the opposition over a political transition.

Britain and France, meanwhile, sought to ratchet up pressure on Assad by raising the prospect of international prosecution of those responsible for human rights abuses during the Syrian government’s 11-month crackdown on demonstrators.

British Foreign Secretary William Hague told the Security Council that his government has sent war-crimes experts to the region to collect evidence of abuses from Syrian refugees who had fled the violence into Jordan, Turkey and elsewhere.France’s foreign minister, Alain Juppé, said his government will press the Security Council to authorize an International Criminal Court investigation into abuses.

Staff writer Colum Lynch at the United Nations contributed to this report.