DAMASCUS: Syrian forces pressed a relentless assault on the protest city of Homs on Wednesday reportedly killing dozens civilians, hours after President Bashar al-Assad said he was committed to ending the bloodshed.

The barrage of gunfire, mortars and shells was launched at dawn and continued all day. State television said a car bomb ripped through the central city, killing and wounding civilians as well as security officers.

Amid a flurry of diplomatic activity, Russian Prime Minister Vladimir Putin insisted any outside intervention to stop the violence would have the destructive effect of "a bull in a china shop."

The United States joined France and Britain in dismissing Moscow's efforts to end nearly 11 months of bloodshed in Syria, and condemned the Syrian regime's brutal crackdown on protesters.

"What is clear is that siding with the Assad regime at this stage will not get Russia anything except for the alienation of the Syrian people," White House spokesman Jay Carney said.

As the regime forces tightened their grip, severing power, communications and other supplies, state media reported "terrorists" attacked Homs' oil refinery.

The authorities frequently blame "terrorists" for attacks on infrastructure, while its opponents accuse the regime of carrying them out to punish centres of resistance.

The Observatory has reported 400 civilians killed since the onslaught on Homs was launched overnight Friday.

It reported a similar deadly onslaught in Zabadani, a restive town near Damascus that has been targeted for seven consecutive days and said 19 people were killed elsewhere in Syria.

In southern Syria, troops used heavy gunfire after an army officer and 17 soldiers defected in Daraa province, cradle of the uprising against Assad's 11 years of iron-fisted rule.

Rights groups estimate more than 6,000 people have died in the regime crackdown on protests since mid-March.

Western and Arab efforts to address the violence have met resistance from Russia, whose foreign minister said after meeting Assad that the Syrian leader was "fully committed" to ending the bloodshed.

Amnesty International added its voice to Western powers urging Russia to use its influence with Syria "to restrain the Syrian military in Homs and ensure it stops using heavy weaponry in residential areas."