Syrian forces killed 50 people yesterday in a sustained attack on Homs, the centre of armed opposition to President Bashar al-Assad, the Syrian National Council opposition group said.

Western countries supporting Assad's downfall were scrambling to find a new diplomatic strategy after failing to win ratification, because of Russian and Chinese vetoes, of a UN Security Council resolution that would have backed an Arab League call for Assad to quit.

The US shut its embassy in Damascus and said all staff had left the country because of the worsening security.

Russia fought back against blistering criticism from the West and Arab states for its veto on Saturday. Foreign Minister Sergei Lavrov, who is due in Damascus today, said condemnations of Moscow's veto had verged on "hysteria".

US President Barack Obama made clear that, however hard Western countries are prepared to lean on Assad diplomatically, they still have no intention of using force to topple him, as they did against Muammar Gaddafi 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 the Today show.

Catherine al-Talli, of the opposition Syrian National Council, said the bombardment of Homs resumed early on Sunday, killing 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.

That attack has been branded a "massacre" by France and "unspeakable" by Obama. It has set the stage for intense efforts to lobby Moscow not to block the UN Security Council resolution.

But Russia argued that 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.

"It is sad that the co-authors decided to put the resolution hastily to a vote, even though we appealed to them with a request to give it a few more days", until after his 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," he said after meeting the foreign minister of Bahrain, one of the Arab states that has sought a tougher stance against Assad.

The opposition Syrian Observatory for Human Rights said it had the names of 29 people killed in Sunday'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 yesterday "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 news agency Sana described attacks in the city by "terrorists".

It said they killed three army officers and abducted several soldiers in Jabal al-Zawiya, in the northern Idlib province.

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