Russia and China vetoed on Thursday a Western-backed UN resolution on Syria over fears that it could lead to foreign military intervention in the violence-wracked Middle East country.

"We offered flexibility on Russia and China's concerns, but still they refused to engage," said Britain's UN envoy Mark Lyall Grant after the vote. "They have chosen to put their national interests ahead of the lives of millions of Syrians."

Lyall Grant also said the failure to approve the resolution would lead to "all-out civil war."

Russia's UN envoy Vitaly Churkin called the comments "unacceptable" and accused the West of thinking only of "its own geopolitical interests, which have nothing in common with those of the Syrian people."

United States envoy to the UN, Susan Rice, called the failure of the reolution "a dark day."

"We have missed yet another critical opportunity to work together," she said. "We, and especially the people of Syria, cannot afford to miss anymore."

Rice also said the conflict in Syria could result in a "proxy war that will engulf the region."

The resolution was drawn up by Britain and threatens non-military sanctions against President Bashar al-Assad's regime if it fails to withdraw heavy weapons and troops from urban areas within 10 days. The resolution is also tied to Chapter 7 of the UN Charter, which provides for the use of force to put an end to the rapidly escalating conflict. It received 11 votes in favor, with two abstentions.

The 15-strong Security Council now has until midnight on Friday to come to an agreement on the extension of the UN monitoring mission in Syria. Russia had earlier circulated a rival resolution that proposed extending the mission by 90 days, but contained no mention of sanctions.

Churkin said on Thursday that Western countries had refused to work with Russia on the resolution and that Moscow would not put it to a Security Council vote.

This was the third time that Russia and China had vetoed a UN resolution on Syria, and Lyall Grant accused both countries after the vote of failing in their responsibilities as permanent members of the UN Security Council.

Russian President Vladimir Putin vowed earlier this year not to allow a repeat of the “Libya scenario" which saw the ouster and murder of long-time Libyan dictator Muammar Gaddafi after a NATO military campaign.

And Russia said on Wednesday after a suicide bombing in Damascus had killed the Syrian defense minister that the adoption of the West’s resolution would be “direct support” for rebel forces, who say they have launched the final battle for control of the capital.

Moscow-based analyst Fyodor Lukyanov said before the vote that a failure to pass the resolution could see the open arming of rebel forces by Western powers.

“If there is no compromise, there will be no more resolutions, and the West, along with leading Arab states, will begin to more actively and more openly support the opposition and hope that Assad will be toppled by force,” said Lukyanov, editor-in-chief of the Russia in Global Affairs journal.

“Certain countries might also recognize the [opposition] Syrian National Council as the legal government,” he added.

The White House on Thursday criticized the vetoes of Russia and China as "highly regrettable" and "highly unfortunate".

White House spokesman Jay Carney told reporters aboard Air Force One: "They are on the wrong side of the Syrian people, the wrong side of hope for peace and stability in the region," White House spokesman Jay Carney told reporters after the vetoes were cast in New York. "It was a highly regrettable decision."

Carney also said the United States does not support extending the U.N. observer mission in Syria after the failure of the Western-backed U.N. resolution.

Fighting raged in Damascus for a fifth day on Thursday, as rumors circulated that Assad and his family had left the country. But Syrian state television aired footage on Thursday afternoon of Assad swearing in the country's new defense minister.

Russia’s Foreign Ministry refused to comment on media reports that Assad’s British-born wife, Asma, was in Moscow when contacted by RIA Novosti on Thursday.

The United Nations, quoting Syrian rights activists, says some 16,000 people have died in Syria since the start of the revolt.