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DAMASCUS (AFP) - Syrian forces rained rockets and shells on protest hubs on Monday and killed at least 66 civilians, activists said, as Washington closed its Damascus embassy and Britain recalled its ambassador.

The opposition Syrian National Council (SNC) said the regime was surrounding Homs with tanks ahead of "a major offensive" and warned of a "genocide" in the central Syrian city.

The Syrian Observatory for Human Rights said at least 42 civilians were killed in Homs alone, and warned that the death toll was likely to rise because many of the dozens of wounded were in critical condition.

State media reported the deaths of three soldiers and said a "terrorist group" blew up an oil pipeline in Homs.

The army also launched an assault on the Zabadani area near Damascus with heavy tank shelling, killing at least three people, said the Britain-based Observatory.

It also reported civilian deaths in Rastan, Hula and Qusair, all towns in Homs province, as well at Sarghaya, near Damascus, in the northern city of Aleppo and in Idlib, northwest Syria.

TIMELINE: Diplomatic moves against Syria

A resident of Homs told AFP the latest assault began shortly after 0400 GMT, with an unprecedented barrages of rockets, mortar rounds and artillery shells.

"What is happening is horrible, it's beyond belief," said activist Omar Shaker, reached by telephone as loud detonations were heard in the background.

"There is nowhere to take shelter, nowhere to hide," he said. "We are running short of medical supplies and we are only able to provide basic treatment to the injured."

One video posted on YouTube apparently showed a field hospital hit by shelling in the Baba Amro district and wounded patients lying on stretchers on the floor amid pools of blood and shattered glass.

Its authenticity could not immediately be verified.

Footage shot by a BBC undercover team in Homs showed buildings ablaze in rebel neighbourhoods as regime forces pounded them with heavy weapons. Plumes of white smoke billowed into the sky.

Damascus blamed the bloodshed in Homs on "terrorist gangs" using mortars.

The violence comes as Western powers seek new ways to punish Damascus amid growing outrage over Saturday's veto by Russia and China of a UN Security Council resolution condemning Syria for a near 11-month crackdown on dissent.

US Secretary of State Hillary Clinton called the veto a "travesty."

White House spokesman Jay Carney warned Syria's allies that backing President Bashar al-Assad was a "losing bet" because his hold on power was "very limited at best."

The State Department said it had closed the American embassy in Syria and withdrawn remaining staff after Damascus refused to address security concerns.

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

The Polish government is to provide emergency consular services to any American citizens remaining in Syria.

US President Barack Obama said it was important to resolve the conflict diplomatically.

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

Britain recalled its ambassador to Syria "for consultations," Foreign Secretary William Hague told parliament.

"We will use our remaining channels to the Syrian regime to make clear our abhorrence at the violence that is utterly unacceptable to the civilised world," Hague said.

French President Nicolas Sarkozy said after meeting German Chancellor Angela Merkel that he would call Russian President Dmitry Medvedev to discuss the international response to the crisis.

Neither France nor Germany, he said, would accept the "blocking" of action on Syria.

Russia and China both defended their vetoes, with Moscow condemning as "hysterical" the West's angry reaction.

Russian Foreign Minister Sergei Lavrov and Foreign Intelligence Service chief Mikhail Fradkov are due in Damascus on Tuesday, as news reports said the mission could try to persuade Assad to quit.

China called on both sides to the conflict to halt the violence that has claimed the lives of at least 6,000 people since March, according to opposition activists.

Saturday's double veto handed Assad's regime a "licence to kill," the opposition SNC charged, urging Syrians around the world "to surround Syrian embassies and stage sit-ins outside them."

The SNC said the "genocide" in Homs showed the regime was "increasing the pace of its crimes and repression."

Saudi Arabia called for "critical measures" on Syria and warned of an impending "humanitarian disaster" after the failure of the UN resolution.

The six-nation Gulf Cooperation Council, of which Riyadh is the leading member, is to meet on Saturday on Syria, on the eve of an Arab League ministerial meeting at the organisation's Cairo headquarters.