Beirut - Syrian forces bombarded Homs on Monday, killing 50 people in a sustained assault on several districts of the city which has become a centre of armed opposition to President Bashar al-Assad, the Syrian National Council opposition group said.

Western countries seeking Assad's downfall scrambled to find a new diplomatic strategy after the defeat of a United Nations Security Council resolution backing an Arab League call for Assad to give up power and start a political transition.

The United States shut its embassy in Damascus and said all staff had left the country due to worsening security. Britain said it withdrew its ambassador from Syria, and would seek further European Union sanctions against Syria.

Russia fought back against blistering criticism from the West for vetoing the resolution on Saturday. Foreign Minister Sergei Lavrov, who is due in Damascus on Tuesday, said condemnations of Moscow's veto had verged on “hysteria”.

US 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.

The opposition Syrian National Council's (SNC) Catherine al-Talli told Reuters shelling of Homs early on Monday killed 50.

Assad's opponents say his tanks and artillery killed more than 200 people in the city on Friday night in the bloodiest incident of the 11-month-old uprising against his rule.

That attack, branded a “massacre” by France and “unspeakable” by Obama, set the stage for intense efforts over the weekend to lobby Moscow not to block the UN Security Council resolution.

But Russia argued the 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.

“It is sad that the co-authors decided to hastily put the resolution to a vote, even though we appealed to them with a request to give it a few more days” until after his own planned trip to Damascus, Lavrov said.

“Some of the voices heard in the West with evaluations of the results of the vote in the UN Security Council on the Syria resolution sound, I would say, improper, somewhere on the verge of hysteria,” Lavrov told reporters after meeting the foreign minister of Bahrain, one of the Arab states that has sought a tougher stance against Assad.

Lavrov has said Russia favours a peace dialogue in Syria that is free of outside interference and preconditions.

He repeated the message in a phone conversation with Arab League chief Nabil Elaraby, the Russian Foreign Ministry said. Elaraby, asked in an interview with Reuters whether Russian mediation could end the crisis, said: “They believe so.”

The collapse of an Arab League monitoring mission in Syria as violence surged set the stage for the UN standoff.

The opposition Syrian Observatory for Human Rights said it had the names of 43 people killed in Monday's bombardment of Homs. Television footage showed smoke rising from buildings, with explosions echoing in the background.

“This is the most violent bombardment in recent days,” said one activist in Syria who was in touch with Homs residents. Another activist said government troops were using multiple rocket launchers in the attack.

Damascus denies firing on houses and says images of dead bodies on the Internet have been staged. State media said on Monday “armed terrorist groups” were firing mortars in the city, setting fire to tyres and blowing up empty buildings to give the impression that Homs was under fire from Assad's forces.

State television quoted an interior ministry official as saying six security personnel died in fighting that killed tens of “terrorists”, and showed brief interviews with Syrians urging the government to strike with a “fist of iron” in Homs.

State news agency SANA described attacks in the city by “terrorists” who it said killed a textile factory worker. It said they also killed three officers and abducted several soldiers in Jabal al-Zawiya in the northern Idlib province.

Reports from activists and authorities are hard to verify because Syria restricts access for independent media.

Activists said there had been explosions in the Khalidiya, Baba Amro, Bayada and Bab Dreib neighbourhoods of Homs.

“They want to drive the Free Syrian Army out,” said Baba Amro resident Hussein Nader by telephone, referring to the force of army deserters and rebels who have held parts of Homs for months. “Rockets are falling seconds apart on the same target.”

Another resident, Omar Shakir, said activists had information the shelling would continue until Thursday, when troops were expected to move into Homs. “We have no one but God - everybody abandoned us,” he said.

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

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

Some observers said the latest offensive by Assad's forces may have been planned for some time, noting a speech last month when he vowed to strike “terrorists” with an iron fist.

“The regime 10 days ago took a decision to confront the opposition in a different manner,” said Ayham Kamel, of the Eurasia Group risk consultancy. “The tactics before were targeted. Now they are set on more direct confrontation with the FSA and opposition activists”. - Reuters