The two powers' refusal to back a US and Arab League plan for Syria came despite the vote coming within hours of the worst single act of violence in the 11-month uprising.

Days of tortuous negotiations led to a final act of brinkmanship as Russia said it could not support a resolution backing a US and Arab League plan for a swift "transition of power" meaning regime change and elections. It was said to have demanded a last-minute change dropping a call, already agreed by Syria in November, for tanks and artillery to be withdrawn from the streets.

That was rejected outright by Western governments after residents of Homs reported an extraordinary bombardment overnight by mortars and heavy artillery. At least 200 were killed according to Syrian anti-government activists, though the numbers could not be independently verified.

"This is a new massacre to add to the other Assad regime massacres," a lawyer and activist in the city who gave his name as Abu Jihad told The Sunday Telegraph. "We ask for international intervention to stop this."

Russians and Chinese diplomats disagree. In the UN resolution they saw another US trick to start yet another war and change the regime of another country also rich in natural resources.