Syrian forces launched a fresh bombardment of powderkeg city Homs at dawn on Monday, killing at least 12 people and wounding many more, activists and correspondents said.
The new assault, which followed what activists said was a "massacre" in the central city overnight Friday and a weekend of further shelling, came after the opposition had warned a veto by Russia and China of a UN Security Council resolution condemning Syria had handed the regime a "licence to kill."
The Syrian Observatory for Human Rights gave an initial toll of at least 12 people killed and dozens wounded early Monday.
Arabic satellite channels broadcast images from Homs which showed plumes of smoke billowing into the sky, as calls to prayer went out from mosques across the battered central city.
A resident of Homs interviewed by AFP said the shelling began shortly after 6:00 am (0400 GMT) with government troops using rockets and artillery fire.
"What is happening is horrible, it's beyond belief," said Omar Shaker, reached by telephone. The sound of gunfire and loud explosions could be heard as he spoke.
Mohammad al-Homsi, a member of the opposition Command Council of the Syrian Revolution, told Al-Jazeera satellite chanel that the Baba Amro sector of Homs had been the main target of Monday's assault.
"Since dawn Baba Amro has been under heavy bombardment. A field hospital was hit and there are many martyrs," he said, adding that shells also landed in other neighbourhoods, including Al-Khalidiya, the main focus of Friday night's onslaught.
Opposition groups and activists reported a "massacre" overnight Friday and early Saturday in Homs, saying more than 200 civilians were killed during a heavy bombardment by regime forces who they said fired tank shells, nail bombs and mortars.
On Sunday, activists reported more shelling in the city, with at least 23 civilians killed.
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The weekend death toll was one of the bloodiest since the uprising against President Bashar al-Assad's regime erupted almost 11 months ago.
Opposition groups say at least 6,000 people have now been killed.
The latest violence comes as Western powers vow to seek new ways to punish Damascus amid growing outrage after the double veto by Russia and China of a UN resolution condemning Syria for its deadly crackdown on protests.
The vetoes on Saturday had handed President Bashar al-Assad's regime a "licence to kill" according to the opposition Syrian National Council.
"The SNC holds both governments accountable for the escalation of killings and genocide, and considers this irresponsible step a licence for the Syrian regime to kill without being held accountable," it said of Russia and China.
The second UN double veto in four months also fuelled angry reaction from Washington, with US Secretary of State Hillary Clinton calling it a "travesty" and vowing to push for new sanctions on Syria.
Russia defended its veto saying Western powers refused to reach a consensus.
Russia's Foreign Minister Sergei Lavrov and Foreign Intelligence Service chief Mikhail Fradkov prepared to visit Damascus on Tuesday, as news reports said the mission could try to push Assad to quit.
"Russia strongly intends to achieve a rapid stabilisation of the situation in Syria through the rapid implementation of much-needed democratic reforms," the Russian foreign ministry said.
In other violence, army deserters destroyed a military control post in the northeast village of Al Bara, killing three officers and capturing 19 soldiers, the Syrian Observatory for Human Rights said Monday.