The United States closed its embassy in Syria and pulled out remaining staff on Monday over serious security concerns as President Bashar al-Assad's regime intensified its bloody crackdown.

US President Barack Obama stressed it was important to resolve the ongoing conflict diplomatically, framing it as very different from the situation in Libya, where Western military intervention helped oust Moamer Kadhafi.

"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 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."

Western powers have vowed to seek new ways to punish Assad's government amid growing outrage over vetoes by Russia and China of a UN resolution condemning Syria for a deadly crackdown that has claimed the lives of more than 6,000 people since March, according to rights groups.

At least 47 people were killed across Syria on Monday as regime troops attacked the flashpoint city of Homs and opened fire in Damascus, Aleppo and Zabadani, activists said.

The UN Security Council vetos on Saturday came hours after the opposition Syrian National Council (SNC) reported a "massacre" in Homs where more than 230 civilians were killed during an assault by regime forces.

"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," the State Department said in its embassy closure 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 Ambassador Ford, departed overland via Jordan on Monday morning.

"The government is getting stretched beyond its ability to control the various elements of violence in the country," one senior official was quoted as saying.

CNN also reported that Secretary of State Hillary Clinton had made the final call to shutter the embassy after the State Department’s management and security staff presented her with their concerns and recommendations.

"This is a decision we never take lightly. Our embassies are a very important part of our diplomacy around the world," a senior official said.

The Syrians were only notified of the decision when all the US staff were out of the country, the report said.