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DAMASCUS (AFP) - Syrian forces pressed a relentless assault on the protest city of Homs on Wednesday reportedly killing 50 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.

Related article: 'God help us' - appeal from Syria's Homs

Rami Abdel Rahman, head of the Syrian Observatory for Human Rights, said at least 69 people were killed across the country on Wednesday, including 50 in Homs alone.

Among those killed in the beleaguered city were three entire families slain overnight by "shabiha" armed regime supporters, he said. The dead included at least three children aged five, seven and 15.

The most intense shelling was in Baba Amr, where at least 23 buildings were completely destroyed, including a home hit by a rocket that killed a little girl, Abdel Rahman said.

Activists in Homs said the widespread shelling was a clear bid to pave the way for a ground assault on Syria's third city.

"Since dawn the shelling has been extremely intense and they are using rockets and mortars," Omar Shaker, reached by satellite telephone from Beirut, told AFP.

"They have destroyed all infrastructure and bombed water tanks and electricity poles. The humanitarian situation is extremely dire and food is lacking.

"We are trying to set up a field hospital but we have no medical supplies."

Ali Hazouri, a doctor in Baba Amr, said a field hospital had been hit and several physicians were wounded, some critically. "One rescuer from the Red Cross had both legs blown off in the shelling."

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.

Sergei Lavrov pointedly declined to say whether Moscow had asked Assad to quit during their talks in Damascus on Tuesday.

"Any outcome of national dialogue should be the result of agreement between the Syrians themselves and should be acceptable to all Syrians," he said.

Putin echoed him.

"Of course we condemn violence from whichever side it comes, but we must not behave like a bull in a china shop. We need to allow people to decide their own fate independently."

But the White House disagreed. "From the (earliest) days of this situation in Syria, there was an opportunity for the Assad regime to engage in dialogue with the opposition," said spokesman Carney.

"Rather than take that opportunity, Assad brutally cracked down on his own people. We don't think that that opportunity is available anymore."

British Prime Minister David Cameron said he had "very little confidence" in the Russian efforts while his Foreign Secretary William Hague spoke to Lavrov by telephone for half an hour to discuss Syria.

And French President Nicholas Sarkozy urged his Russian counterpart to give full support to an Arab League peace plan to persuade Assad to step down.

But Medvedev "called on partners to avoid hasty, unilateral steps," in the Syria crisis, the Kremlin said in a statement following a phone conversation between the two leaders.

Turkey, meanwhile, said it was planning an international conference of regional players and world powers on solving the crisis "as soon as possible."

Moscow vetoed along with China a UN resolution condemning the crackdown last weekend and has staunchly stood by its last ally in the region, a key buyer of military hardware that hosts a strategic Russian naval base.

UN rights chief Navi Pillay said the failed Security Council resolution "appears to have fuelled the Syrian government's readiness to massacre its own people in a bid to crush dissent."

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."