Russia and China on Saturday vetoed a UN Security Council resolution condemning the Syrian government's murderous crackdown on protests for the second time.

Western governments reacted with fury to the new block on UN action over President Bashar al-Assad's 10 month-old assault on demonstrators which followed weeks of acrimonious negotiations over the text.

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 15-nation council. UN leader Ban Ki-moon said the failure to agree a resolution "undermines" the United Nations.

Thirteen countries voted for the resolution drafted by Arab and European nations which would have given strong backing to an Arab League plan to end the crisis.

Russia and China made a repeat of their rare double veto carried out on October 5 on an earlier condemnation of Assad. Russia's UN envoy Vitaly Churkin called the draft resolution "unbalanced."

The Security Council has now only agreed one statement, which has a lower standing, on the Syrian crisis since protests erupted in March last year.

After the earlier veto, western nations said they made substantial concessions taking out references to an arms embargo, Arab League sanctions and backing for Arab League calls for Assad to hand over power to a deputy.

Diplomats said Russia demanded new changes to the text on Saturday morning so that the withdrawal of Syrian troops from cities should be linked to an end to attacks by opposition groups. It was "unbelievably cynical," said one European diplomat.

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

Western nations backing the resolution had since the start of the Syria crisis been "undermining the opportunity for a political settlement, calling for regime change, pushing the opposition towards power," he added.

Li Baodong, China's UN representative, backed Russia's call for new changes. "To push through a vote when parties are still seriously divided over the issue will not help maintain the unity and authority of the Security Council, or help resolve the issue," he told the council.

But there was widespread condemnation of the new veto from the European and Arab countries behind the resolution, the United States and UN leadership.

"I would like to express our great regret and disappointment" at the veto, said Morocco's UN ambassador Mohammed Loulichki, whose country is the Arab member of the 15-member council.

Western ambassadors highlighted the concessions made to Russia in weeks of negotiations on the draft text.

"It is a disgrace for the council," said Germany's UN envoy Peter Wittig.

France's UN ambassador Gerard Araud noted how the debate was held on the thirtieth anniversary of a massacre in the Syrian city of Hama in which tens of thousands died under Assad's father, Hafez.

"Father and son are killing; it would seem to be hereditary in Damascus," he said.

India and South Africa which abstained in the October vote, backed the latest resolution. Pakistan was also among council members to back the resolution.

UN leader Ban Ki-moon deeply regrets the failure to agree a resolution, said his spokesman Martin Nesirky.

"It undermines the role of the United Nations and the international community in this period when the Syrian authorities must hear a unified voice calling for an immediate end to its violence against the Syrian people," Ban said.

Many ambassadors predicted that a new effort would be made to agree UN action.

India's UN envoy Hardeep Singh Puri said Russia had wanted to wait three days for a vote "but with the spiralling violence, the council was not in the mood."

He told reporters that the message of the 13-2 vote was clear: "That text is not going to go away."