UNITED NATIONS: Russia and China on Saturday vetoed a UN Security Council resolution condemning Syria for its lethal crackdown, hours after Syrian forces bombed the protest city of Homs killing hundreds of people.

The heaviest reported day of death since the Syrian uprising began coupled the second such UN veto in four months, triggered a wave of international outrage, with Washington saying it was "disgusted" with the veto and France calling the massacre a "crime against humanity."

President Bashar 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.

AFP was not able to verify the authenticity of videos or the tolls because of restrictions on reporting in Syria. But 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.

In a separate message on Twitter, Rice wrote: "Disgusted that Russia and China prevented the UN Security Council from fulfilling its sole purpose."

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

UN chief Ban Ki-moon said the failure to agree a resolution "undermines" the United Nations.

But Russia's UN envoy Vitaly Churkin defiantly rejected attacks from the European and Arab nations that proposed the resolution, which has been negotiated for several weeks. He said it was "unbalanced" and that the western backers only wanted "regime change" in Damascus.

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.

US Secretary of State Hillary Clinton said she had tried to negotiate with Russian Foreign Minister Sergei Lavrov on the sidelines of the Munich Security Conference in Germany.

But the resolution's backers said they decided to end negotiations and call for a vote after Russia demanded new changes on Saturday morning.

Russia -- for whom Syria is its last remaining major ally in the Middle East -- announced that Lavrov and intelligence chief Mikhail Fradkov would travel to Damascus on Tuesday to press Assad to discuss a political solution.

China said it backed Russia's demand for changes to the UN resolution and Lavrov's visit.

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

The SNC said it took over the embassy in Tripoli.

The Homs attack came on the 30th anniversary of a massacre by Assad's father Hafez in the city of Hama in which tens of thousands died.

Homs was not the only city to suffer in the recent explosion of violence.

Assad's forces also "bombed" the northern town of Jisr al-Shughur near the Turkish border, and suburbs of Damascus, the Britain-based Observatory said.

Twelve people were killed when security forces opened fired on a funeral procession in Daraya, outside Damascus, the Observatory's Abdel Rahman said.