Russia and China blocked a UN Security Council resolution condemning Syria for its crackdown on protests, amid growing outrage Sunday at a "massacre" in the protest city of Homs and a spiralling death toll.

The double vetoes on Saturday drew swift condemnation from world powers while the opposition Syrian National Council said it gave the regime of President Bashar al-Assad a "licence to kill."

On the ground, activists on Sunday reported another 57 people killed in Syria, adding to the body count of one of the bloodiest weekends since the uprising against Assad's regime erupted almost 11 months ago.

Activists and residents had reported more than 200 civilian deaths, including women and children, overnight Friday during a massive assault by regime forces in the central flashpoint of Homs.

The surge of violence coupled with the second UN veto in four months triggered a wave of international outrage at the failure to reach a common stand at the United Nations.

Washington said it was "disgusted" with the rare double veto and France denounced Friday's massacre in the city of Homs as a "crime against humanity."

Assad's troops shelled Homs "randomly" during the night, killing men, women and children, the opposition Syrian National Council (SNC) said.

It said at least 260 civilians were killed in the onslaught. The Syrian Observatory for Human Rights said about 100 women and children were among the 237 dead in its toll. Both said hundreds more were wounded.

The Assad regime "committed one of the most horrific massacres since the beginning of the uprising in Syria," the SNC said. Opposition groups say more than 6,000 people have now been killed in the country since last March.

Dozens of bodies and scenes of chaos could be seen in video images shown by the Al-Jazeera and Al-Arabiya television channels.

Church bells rang out and Muslim prayers were recited in Homs mosques for those killed, activists said. Thousands took part in funeral processions across the city on Saturday.

The authenticity of the figures and videos are difficult to independently confirm because of reporting restrictions imposed on the foreign media.

US President Barack Obama denounced the "unspeakable assault" and demanded that Assad step down.

"Assad must halt his campaign of killing and crimes against his own people now.He must step aside and allow a democratic transition to proceed immediately," Obama said in a statement.

The Syrian government denied responsibility for the deaths, blaming them on opposition rebels seeking to influence Security Council debate on Syria. But Russia and China used their diplomatic muscle for the second time in four months to block a resolution condemning the violence.

The other 13 countries in the 15-member council voted for the resolution, proposed by European and Arab nations to give strong backing to an Arab League plan to end the crackdown.

Russia and China "remain steadfast in their willingness to sell out the Syrian people and shield a craven tyrant," US ambassador Susan Rice told the council.

Britain is "appalled" at the veto, said its UN ambassador Mark Lyall Grant, while French President Nicolas Sarkozy "strongly deplores" the veto by Russia and China, his office said.

Russia's UN envoy Vitaly Churkin justified the veto by saying the proposed resolution "sent an unbalanced signal to the Syrian parties."

His Chinese counterpart Li Baodong said pushing through such "a vote when parties are still seriously divided... will not help maintain the unity and authority of the Security Council, or help resolve the issue."

Western envoys said they had bent over backwards to change the text after Russia had balked at any resolution that could be used to justify foreign military intervention, called for Assad to quit or imposed an arms embargo on Syria.

The umbrella Syrian National Council said in a statement that "Syrians and others around the world" had looked to the Security Council to issue a strongly worded resolution.

"The SNC holds both governments accountable for the escalation of killings and genocide, and considers this irresponsible step a licence for the Syrian regime to kill without being held accountable," it said.

As news of the Homs killing spread, protesters stormed Syrian embassies in Athens, Berlin, Cairo, Kuwait, London and Sydney. Tunisia said it was expelling Syria's ambassador and withdrawing its recognition of the Assad government.

On Sunday, Tunis urged other Arab nations to follow its lead.

The Syrian Observatory for Human Rights, meanwhile, on Sunday reported 57 more people killed in Syria, including 27 soldiers and six armed rebels.

Among the dead were 12 civilians killed Saturday when security forces opened fire in the Damascus suburb of Daraya on mourners attending funerals of people killed the previous day, the Britain-based rights watchdog said.

It said nine Syrian soldiers died and 21 were wounded in clashes overnight with armed rebels at Jebel Al-Zawiya in Idlib province, which borders Turkey.

The Turkey-based Free Syrian Army, which comprises army deserters and armed volunteers, regularly attacks the security forces in a bid to halt their brutal crackdown on dissent which rights groups say has killed more than 6,000 people since mid-March.

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