US Secretary of State Hillary Clinton warned her Russian counterpart on Saturday that Syria risks plunging towards civil war if Moscow failed to back UN moves to stop the bloodshed.

Clinton's remarks came just as Russia and China Saturday vetoed a UN Security Council resolution condemning the deadly crackdown by President Bashar al-Assad's government on protests for the second time.

"What more do we need to know to act decisively in the Security Council?" Clinton asked after she held talks with Russian Foreign Minister Sergei Lavrov on the sidelines of the Munich Security Conference.

She talked of a "living nightmare" playing out in the city of Homs where more than 200 people were reported killed.

In a swipe at Russia and China, Clinton, recalling a stance she had taken last week, said "to block this resolution is to bear responsibility for the horrors that are occurring on the ground in Syria."

The chief US diplomat said she had tried to bridge differences over the UN resolution during a 45-minute meeting with Lavrov.

Offering to be constructive, "I thought there might be some ways to bridge even at the last moment a few of the concerns that the Rusians had," she said. "That has not been possible."

She said Lavrov had asked her what the "end game" was in a resolution he felt failed to deal equally with armed groups fighting the Assad regime.

"Well, the endgame, in the absence of us acting together as the international community, I fear is civil war," Clinton said.

"The potential endgames, if we are serious about putting this kind of international pressure on the Assad regime... is the possibility of beginning a transition similar to what we've seen now beginning in Yemen," she said.

After months of deadly protests, Yemeni President Ali Abdullah Saleh finally signed a power transfer deal in November that effectively ended his three decades in power. He has now begun carrying it out.

As every day passes without international action, the risk of civil conflict grows in Syria, she warned.

"The Syrian people have asked the Security Council to act. The Arab League has asked the Security Council to act.... We should act now," she said.

Some 30 years after the Hama massacre perpetrated by his late father Hafez al-Assad, Clinton said "the international community must send (Bashar) Assad a clear message: 'by repeating the horrors of Syria's past, you have lost your place in Syria's future.'"

Asked if there would be any possibility of military intervention, Clinton replied: "No, military intervention has been absolutely ruled out. And we've made that clear from the very beginning."