More than 80 people were killed in Syria Thursday, most of them in a relentless blitz on the city of Homs, an attack US President Barack Obama decried as "outrageous bloodshed".

Shelling erupted at daybreak, killing more than 50 civilians in the besieged central city and burning several bodies beyond recognition, the Britain-based Syrian Observatory for Human Rights said.

At least 83 people were killed across the country on Thursday, said the Britain-based Observatory.

Troops trying to crush opponents of President Bashar Al-Assad have killed at least 400 people in a six-day onslaught on Homs, opposition activists say.

"The shells are raining down on us and regime forces are using heavy artillery," said Ali Hazuri, a doctor in the Baba Amr district reached by telephone from Beirut.

Omar Shaker, an activist in Baba Amr also reached by phone, said residents were hiding on ground floors as there were no underground shelters.

"When you venture outside, you can see craters every 10 metres (yards)," he said.

British Prime Minister David Cameron said Assad's regime appeared determined to kill its own people.

"It's quite clear that this is a regime that is hell-bent on killing, murdering and maiming its own citizens," Cameron told reporters in Stockholm. "It really is appalling, the scenes of destruction in Homs."

He called for "transition and change in Syria."

Foreign Secretary William Hague stressed that Britain has no plans to help arm Syria's opposition.

But Turkish Foreign Minister Ahmet Davutoglu said Thursday during a visit to Washington that the international community cannot afford to watch the "massacre" taking place in Syria without acting.

Davutoglu is urging an international conference to resolve violence that erupted when demonstrators began demanding last spring that Assad be removed from office.

"We cannot let Syrian people die every day and the international community will follow blindly," Davutoglu said during a lecture at George Washington University in the US capital.

Germany meanwhile backed a proposed joint Arab League-UN mission to monitor the Syrian government's deadly crackdown on protests, but other major powers were more cautious.

Prospects for the mission that the Arab League chief has proposed to UN leader Ban Ki-moon could depend on an Arab League foreign ministers' meeting this weekend and the backing of the major powers.

As the international community struggles to find a new diplomatic response to Assad's assault on protest cities, Germany's Foreign Minister Guido Westerwelle gave the strongest welcome to the Arab League-UN proposal.

"In addition to the establishment of a contact group of 'friends of a democratic Syria' we must also undertake a new attempt to resolve the crisis through the United Nations," Westerwelle said in Berlin.

He said a joint Arab League-UN observer mission and the naming of a UN special representative on the Syria conflict "would serve peace and balance. And both would be a clear signal by the international community to the Assad regime."

Ban said Arab League chief Nabil El-Araby had told him Tuesday that he would be sending observers back to Syria and wanted it to be a joint UN-Arab League operation. El-Araby also wanted a joint UN-League special envoy to Syria.

EU foreign affairs chief Catherine Ashton said Russia must confront "the reality" of the deadly crackdown in Syria, after Moscow vetoed a UN resolution condemning the violence.

"My message to my Russian colleagues is they too need to recognize the reality of the situation on the ground" in Syria, said Ashton, who was making a two-day visit to Mexico.

Obama, in comments after White House talks with Italian Prime Minister Mario Monti, condemned "the outrageous bloodshed that we've seen", and urged "a transition from the current government that has been assaulting its people."

Ban hit out at Russia and China for their steadfast refusal to back UN resolutions condemning the violence in Syria, saying this had encouraged Assad's regime to continue its repression.

Moscow, a staunch ally of Damascus, has insisted that any solution to end nearly one year of bloodshed must come from within Syria.

However the United States, France and Britain have dismissed such arguments while piling pressure on Moscow to change tack.

Despite the bloodshed, activists urged Syrians to turn out for a big demonstration against Russia on Friday, a traditional day of protests that follow the main weekly Muslim prayers.

"Russia is killing our children. Its planes, tanks and veto are also killing our children," said a banner on the Facebook page of The Syrian Revolution 2011.

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