Russia and China have vetoed a UN Security Council resolution calling for Syrian president Bashar al-Assad to step down, in a vote that went against 13 council members and left the U.S. expressing disgust over the result.

The rare double-veto on the weekend comes in the wake of what activists have described as the most severe attack against opposition groups in 11 months of unrest, with the death toll in the city of Homs estimated to have exceeded 200.

Reacting to the vote on Saturday, U.S. Ambassador Susan Rice accused China and Russia of helping to prop up "a craven tyrant" and "sell out the Syrian people," adding that the United States was "disgusted" by the vetoes.

"For months this council has been held hostage by a couple of members," Rice said. "These members stand behind empty arguments and individual interests while delaying and seeking to strip bare any text that would pressure Assad to change his actions.

"Any further bloodshed that flows will be on their hands," she added.

There has been increasing international pressure to stop Assad's regime, which began a bloody crackdown last March.

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Supporters of the Free Syrian Army chant slogans during a protest in Reef Damascus, north of Damascus, early Saturday. (Reuters)

Russia’s veto was expected since it has military connections to the Assad regime, CBC reporter David Common said from New York.

"China comes at it from a different reason," said Common at the UN on Saturday. "Perhaps it is going in there to try to cushion some of the blow for Russia."

Common said that without China's support, Russia would have had to stand alone — especially against allies of the Arab League, the 22-nation organization that had put forth the resolution.

UN Secretary-General Ban Ki-moon called it a "great disappointment to the people of Syria and the Middle East, and to all supporters of democracy and human rights."

Described as 'massacre'

The world body's failure to pass the resolution comes after U.S. President Barack Obama condemned the latest atrocrities in Syria on Saturday and told its citizens: "We are with you, and the Assad regime must come to an end."

The British-based Syrian Observatory for Human Rights described the most recent assault by Syrian forces as a "massacre" in which as many as 260 were killed and dozens of homes destroyed in a residential neighbourhood.

"We've been hearing unconfirmed reports about how the city's residents have been digging bodies out of the rubble with their bare hands as the emergency services, we're told, were unable to help," reported CBC's Dominic Lavitis, speaking from London.

He said Syrian forces shelled the area and rolled out in tanks to fire mortar rounds.

Rights groups blast UN's 'betrayal'

Amnesty International has called the action by Russia and China a "shockingly callous betrayal of the people of Syria."

"This is a completely irresponsible use of the veto by Russia and China," said Salil Shetty, Amnesty International's Secretary-General in a statement released Saturday.

"It is staggering that they have blocked the passage of what was already a very weak draft resolution."

In his statement, Obama spoke out against the "relentless brutality" of Assad's government and again urged the Security Council to stop the killings.

"The international community must work to protect the Syrian people from this abhorrent brutality," he said.

Baird blasts UN's 'paralysis'

Canada's Foreign Affairs Minister John Baird said the UN's "paralysis of power is particularly deplorable given the reported upsurge in violence overnight in Homs, which we condemn without reservation."

"Today's failure by the UN Security Council to effectively deal with the crisis in Syria is yet another free pass for the illegitimate Assad regime and those backing it," Baird said in a statement issued Saturday.

"Canada is disappointed in the extreme."

Activists say the number killed in shelling that began late Friday ranged from 217 to 260, making it the deadliest attack in the central city since the uprising against President Bashar Assad's regime began 11 months ago.

Some activists said the violence was triggered by a wave of army defections in Homs. There have been reports that army defectors have set up checkpoints in the area.

The Syrian Observatory for Human Rights and the Local Co-ordination Committees said more than half of the killings from shelling that began late Friday were reported in the Khaldiyeh neighbourhood.

The opposition Syrian National Council described it as "one of the most horrific massacres since the beginning of the uprising in Syria."

The reports could not be independently confirmed and the government has accused the opposition groups of engaging in a propaganda campaign.

'Sad day for Syrians'

Russia's Foreign Minister Sergey Lavrov had been quoted, just prior to the vote on Saturday, as saying that consensus was still possible. In comments made to local media on Saturday, he urged the West to accommodate Russian concerns to reach a compromise.

Embassies attacked

Demonstrators have stormed five Syrian embassies in Europe and the Middle East in light of the Homs killings.

Protesters were able to break windows or force their way in at embassies in London, Athens, Berlin, Cairo and Kuwait.

Lavrov already warned Washington that any attempt to put a draft resolution on Syria to vote at the United Nations would lead to "scandal," a blunt warning that Moscow is prepared to use its veto power. He said Moscow had submitted its amendments to the Western-backed draft.

Lavrov said Moscow still sees two problems of "crucial importance" with the draft UN resolution. He said it makes too few demands of armed groups opposing the Assad regime. He also said Moscow remains concerned about whether it prejudges the outcome of a national dialogue among political forces in Syria.

Russia and China, which wield veto power at the Security Council, have blocked previous Western attempts to impose sanctions on the Syrian regime over its crackdown on protests.

"It is a sad day for this council, a sad day for Syrians and a sad day for all friends of democracy," French Ambassador Gerard Araud said after the resolution was vetoed.

Araud said Russia and China had "made themselves complicit in a policy of repression carried out by the Assad regime."

"Today the Security Council has failed to live up to its responsibility," German Ambassador Peter Wittig said. "The people in Syria have been let down again."

The UN has said that more than 5,400 people have been killed in violence since March. Hundreds more have been killed since that tally was announced, and activists say 200 died in the city of Homs on Saturday.

With files from The Associated Press