Russia and China vetoed on Saturday a UN resolution that backed an Arab plan calling on Syrian President Bashar Al-Assad to quit, stalling global efforts to end his bloody crackdown on unrest after hundreds were reported killed in the city of Homs.

The high-level diplomatic setback came after world leaders and Syrian opposition activists accused Al-Assad's forces of a massacre in a sustained shelling of Homs, the bloodiest episode in 11 months of upheaval in the pivotal Arab country.

Russia and China joined in a double veto of a Western- and Arab-driven resolution at the UN Security Council endorsing the Arab League plan for Al-Assad to hand power to a deputy to make way for a transition towards democracy.

The other 13 council members voted for the resolution that would have said the council "fully supports" the League plan aimed at stopping Syria's bloodshed, whose sectarian overtones threaten stability in the wider Middle East region.

Russia complained that the draft resolution was an improper and biased attempt at "regime change" in Syria, which is Moscow's sole major Middle East ally, an important buyer of Russian arms exports and host to a Russian naval base.

With an eye to events in Homs, US Ambassador to the United Nations Susan Rice dispensed with the usual diplomatic courtesies and declared she was "disgusted" by the Russian-Chinese veto, adding that "any further bloodshed that flows will be on their hands."

Shortly before the Security Council voted, US President Barack Obama denounced the "unspeakable assault" on Homs, demanded that Al-Assad leave power immediately and called for UN action against Al-Assad's "relentless brutality."

"Any government that brutalises and massacres its people does not deserve to govern," Obama said.

He and other Western and Arab leaders exerted unprecedented pressure on Russia to allow the Security Council to pass the Arab League-backed resolution that calls for Al-Assad to relinquish his autocratic powers and end the violence. The world body says over 5,000 civilians have been killed.

But Russia, and China following Moscow's lead, weighed in to torpedo UN action on Syria for the second time in four months. In October, they vetoed a European-drafted resolution condemning Syria and threatening it with possible sanctions.

US Secretary of State Hillary Clinton said it had not been possible to work constructively with Russia ahead of the vote, even though military intervention in Syria - fiercely opposed by Moscow - had been absolutely ruled out.

"I thought that there might be some ways to bridge, even at this last moment, a few of the concerns that the Russians had. I offered to work in a constructive manner to do so. That has not been possible," she told reporters at a Munich conference.

Clinton warned that the risk of more bloodshed and civil war in Syria had risen after the collapse of the U.N. resolution.

French President Nicolas Sarkozy said after the vote that France was consulting with Arab and European countries to create a contact group on Syria to try to find a solution to the crisis.

"France is not giving up," Sarkozy said in a statement, saying Paris was in touch with Arab and European partners to create a "Friends of the Syrian People Group" that would marshal international support to implement the Arab League plan.

The uprising pits Syria's majority Sunni Muslims against Al-Assad's minority Alawites, an offshoot of Shi'ite Islam, who have dominated the country's power structure for decades.

After what US officials called "vigorous" talks between Clinton and Foreign Minister Sergei Lavrov, Moscow announced that he and its foreign intelligence chief would fly to Syria on Tuesday to meet Al-Assad, although the trip's goal was not given.

Moscow objected that the resolution contained steps against Al-Assad, but not against his armed opponents, Lavrov said in Munich before the vote. "Unless you do it both ways, you are taking sides in a civil war."

In New York, Western delegations rejected what they called "wrecking amendments" by Russia to add language blaming the opposition along with the government for violence and diluting calls for Syria to withdraw its security forces from cities.

Russian Ambassador Vitaly Churkin denied that Moscow's amendments were last-minute, or that Russia was standing in the way of a peaceful resolution of the crisis.

"Some influential members of the international community, unfortunately, including those sitting around this table, from the very beginning of the Syrian process have been undermining the opportunity for a political settlement," Churkin said.

Syrian UN envoy Bashar Ja'afari criticised the resolution and its sponsors, which included Saudi Arabia and seven other Arab states, saying nations "that prevent women from attending a soccer match" had no right to preach democracy to Syria.

He also denied that Syrian forces killed hundreds of civilians in Homs, saying that "no sensible person" would launch such an attack the night before the Security Council was set to discuss his country.

Residents of Homs' battered Baba Amro district, speaking by telephone, denounced the Russian-Chinese veto, some chanting, "Death, rather than disgrace."

One resident who identified himself as Sufyan said: "Now we will show Al-Assad. We're coming, Damascus. Starting today we will show Al-Assad what an armed gang is." Al-Assad has called his opponents "armed gangs" and "terrorists" steered from abroad.

Mohammed Loulichki, the UN ambassador of Morocco, the sole Arab member of the 15-nation council, voiced his "great regret and disappointment" at the veto and said the Arabs had no intention of abandoning their plan. British envoy Mark Lyall Grant said there would be a new UN push if violence continued.

France called the Homs assault a "massacre" and a "crime against humanity."

Turkey said hundreds had been killed and the United Nations must act. Tunisia expelled the Syrian ambassador and the flag above its embassy was brought down.

Death tolls cited by activists and opposition groups ranged from 237 to 260, making the Homs attack the bloodiest so far in Al-Assad's crackdown on protests and one of the deadliest episodes in the "Arab Spring" of revolts that have swept the region.

Residents said Syrian forces began shelling the Khalidiya neighbourhood at around 8 p.m. (1800 GMT) on Friday using artillery and mortars. They said at least 36 houses were completely destroyed with families inside.

"We were sitting inside our house when we started hearing the shelling. We felt shells were falling on our heads," said Waleed, a resident of Khalidiya.

"The morning has come and we have discovered more bodies, bodies are on the streets," he said. "Some are still under the rubble. Our movement is better but there is little we can do without ambulances and other things."

An activist in the neighbourhood contacted by Reuters said: "We have dug out at least 100 bodies so far, they are placed in the two mosques." He put the total number of wounded at 500.

Video footage on the Internet described as being from Homs showed at least eight bodies assembled in a room, one of them with the top half of its head blown off. A voice on the video said the bombardment was continuing as the footage was filmed.

As news of the violence spread, angry crowds of Syrians stormed their country's embassies in Cairo.