The United States closed its embassy in Syria and pulled out all its staff on Monday, but President Barack Obama shied away from talk of military intervention and vowed to pursue diplomatic means.

The closure came amid deteriorating security as President Bashar al-Assad's government intensified its bloody crackdown, raining rockets and shells on protest hubs in a fresh onslaught that killed at least 66 civilians.

Britain and Belgium recalled their envoys as Western powers sought new ways to punish Damascus amid growing outrage after Russia and China on Saturday vetoed a UN resolution condemning Syria for its 11-month crackdown on dissent.

"The United States has suspended operations of our embassy in Damascus as of February 6. Ambassador (Robert) Ford and all American personnel have now departed the country," a State Department statement said.

"The recent surge in violence, including bombings in Damascus on December 23 and January 6, has raised serious concerns that our embassy is not sufficiently protected from armed attack," it said, referring to attacks linked to Al-Qaeda.

"We, along with several other diplomatic missions, conveyed our security concerns to the Syrian government but the regime failed to respond adequately."

Obama said a negotiated solution with Syria was still possible and defended his administration's handling of the crisis, saying the US had been "relentless" in demanding that Assad leave power.

"It is important to resolve this without recourse to outside military intervention and I think that's possible," he said in an NBC interview broadcast Monday.

"My sense is you are seeing more and more people inside of Syria recognizing that they need to turn a chapter and the Assad regime is feeling the noose tightening around them. This is not a matter of if but when."

His administration hit out angrily at Moscow and Beijing for blocking a UN Security Council resolution condemning Syria for a crackdown that has claimed the lives of more than 6,000 people, according to rights groups.

"Russia and China will, I think, come to regret this decision which has aligned them with a dying dictator, whose days are numbered, and put them at odds with the Syrian people and the entire region," Washington's UN ambassador Susan Rice said on Monday.

She added that the vote, which came just hours after Syrian forces bombed the city of Homs, killing hundreds of civilians, "put a stake in the heart of efforts to resolve this conflict peacefully."

The resolution on the table "would have given political backing to an Arab League plan to begin a negotiated transition," Rice told CNN.

US Secretary of State Hillary Clinton had already called the double veto on the Syrian resolution a "travesty" and chastised those who opposed the vote for protecting Assad's regime.

White House spokesman Jay Carney followed up on Monday by warning Syria's allies that backing Assad was a "losing bet" because the Syrian leader's hold on power was "very limited at best."

"The deteriorating security situation that led to the suspension of our diplomatic operations makes clear once more the dangerous path Assad has chosen and the regime's inability to fully control Syria," State Department spokeswoman Victoria Nuland said in a written statement.

"It also underscores the urgent need for the international community to act without delay to support the Arab League's transition plan before the regime's escalating violence puts a political solution out of reach and further jeopardizes regional peace and security."

Senior State Department officials told CNN that two embassy employees left by air last week and 15 others, including Ford, departed overland via Jordan on Monday morning.

The Polish government was to provide emergency consular services to any American citizens remaining in Syria, but the State Department stressed that Ford remained the ambassador.

He "will maintain contacts with the Syrian opposition and continue our efforts to support the peaceful political transition which the Syrian people have so bravely sought," a statement said.