(Reuters) - A Syrian regime military assault on Homs killed dozens of people on the eve of a visit to Damascus by Russia's Foreign Minister Sergei Lavrov aimed at pressing President Bashar al-Assad to end an 11-month uprising by implementing swift reform.

The violence Monday came as world powers scrambled for a diplomatic strategy after the defeat of a U.N. Security Council resolution backing an Arab League call for Assad to give up power and start a political transition.

A member of the main opposition Syrian National Council said Assad's forces killed 50 people in a sustained bombardment of Homs, a center of armed opposition to his rule, two days after activists reported 200 people were killed in shelling.

Syrian authorities, who have denied firing on houses, said security forces killed tens of terrorists in Homs Monday morning. An Interior Ministry statement said six members of the security forces were killed in the clashes.

The United States shut its embassy in Damascus and said all staff had left the country due to worsening security.

Belgium and Britain recalled their ambassadors from Syria, and London said it would seek further European Union sanctions against the country.

U.S. President Barack Obama said that, however hard Western countries are prepared to lean on Assad diplomatically, they still had no intention of using force to topple him, as they did against Muammar Gaddafi in Libya last year.

I think it is very important for us to try to resolve this without recourse to outside military intervention. And I think that's possible, he told NBC's Today show.

Russia fought back against blistering criticism from the West for blocking the U.N. resolution Saturday. Foreign Minister Lavrov said condemnations of Moscow's veto had verged on hysteria.

He was heading to Syria Tuesday because Moscow sought the swiftest stabilization of the situation in Syria on the basis of the swiftest implementation of democratic reforms whose time has come, Lavrov's ministry said.

SWIFT REFORMS

Assad has promised political reforms including a new constitution followed by a parliamentary election, but has also pledged to crush terrorists he blames for the violence.

Syria's opposition, which rejected a Russian invitation for talks with Syrian officials in Moscow, says Assad's promised reforms are not credible after his crackdown on protests in which the United Nations says 5,000 people have been killed.

In addition to months of national unrest, the Syrian capital was hit by suicide bombings in December and January.

Arab League Secretary-General Nabil Elaraby told Reuters he spoke to Lavrov Monday and said the foreign minister would present an initiative to Damascus. He gave no details and when asked if he thought it could end the crisis, he replied: They believe so.

Russia, seeking to retain a foothold in the Middle East centered on its military ties with Damascus, may be torn between trying to bolster Assad and seeking his exit. It also could take a middle path, trying to buy time by counseling the government to make some concessions and reduce the bloodshed.

I think that now, after Russia imposed a veto, Lavrov (is) travelling to tell Assad that we did everything possible, said Fyodor Lukyanov, editor of the journal Russia in Global Affairs.

Now the main task for Lavrov is to tell Assad that if there is no visible change in Syria, then regardless of the Russian position he should be bracing for external military measures, Lukyanov said.

Russia argued that Saturday's draft U.N. resolution was one-sided and would have amounted to taking the side of Assad's opponents in a civil war. China also vetoed the measure, by most accounts following Russia's lead.

Some of the voices heard in the West with evaluations of the results of the vote in the U.N. Security Council on the Syria resolution sound, I would say, improper, somewhere on the verge of hysteria, Lavrov said.

A chorus of European officials - and U.S. Secretary of State Hillary Clinton, who called the veto a travesty - condemned Russia and China in terms unusually harsh by diplomatic norms and said they would bear responsibility for future bloodshed.

French Defense Minister Gerard Longuet said on Monday: There are political cultures which deserve a kick in the ass ... To accept that a dictator can operate freely is disgraceful for governments that accept it.

Qatar's Prime Minister Sheikh Hamad bin Jassim, who leads the Arab League's ministerial committee on the Syrian crisis, told Al Jazeera he was shocked by Moscow and Beijing's stance and called for clear steps to stop the bloodbath in Syria.

Clinton said Sunday the United States would work with other nations to try to tighten sanctions against Assad's government and deny it arms in the absence of a U.N. resolution.

Activists said a blast hit an oil pipeline feeding a main refinery in Homs Monday, the second attack in a week. Separately, three people died when the opposition-held town of Zabadani, near the Lebanese border, came under fire Monday.

Local fighters said they would hit regime targets if the army did not pull back by Tuesday morning.

Syrian army defectors announced they were organizing a new Higher Revolutionary Council to supersede the Free Syrian Army as the main armed force battling Assad's rule. The new body would be commanded by Gen. Ahmed al-Sheikh, the highest-ranking officer to defect to Turkey from government forces.

(Additional reporting by Khaled Yacoub Oweis in Amman, Joseph Logan in Beirut, Gleb Bryanski and Steve Gutterman in Moscow; Editing by Andrew Heavens and Michael Roddy)