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

The vetoes came as activists reported another 47 people killed in Syria, adding to the body count of one of the bloodiest weekends since the uprising against President Bashar al-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.