More than 80 people were killed in Syria Thursday, most of them in a new regime 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.
Troops trying to crush opponents of President Bashar al-Assad have killed at least 400 people in a relentless 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.
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."
The Observatory said 11 people died when a shell hit their house in the Homs neighbourhood of Inshaat, while six were killed in Rastan in the same province.
Elsewhere, rebels killed seven security forces when they ambushed two buses near the southern town of Daraa, the cradle of the revolt, said the Observatory.
But in apparent reference to the same incident, SANA state news agency reported the authorities had made arrests as they foiled an attempt to steal a truck transporting cars.
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Republican US Senator John McCain said in Washington some 40,000 Syrian troops had quit the regime crackdown, a figure he said visiting Turkish Foreign Minister Ahmet Davutoglu had provided.
"Some of them have gone home, some of them have joined" rebel forces, McCain told reporters.
UN chief Ban Ki-moon said late Wednesday that the "appalling brutality" of the assault on Homs "is a grim harbinger of worse to come".
He also launched the idea of sending a joint observer mission with the Arab League but it received a lukewarm reception from Western powers.
The head of the pan-Arab bloc, which suspended its month-long monitoring mission to Syria on January 28, spoke with Ban about the proposed mission.
The UN chief said consultations would be held with the Arab League and Security Council members in coming days "before fleshing out the details".
France said there had to be "guarantees" for the mission. Germany called it a "very serious" idea, but also said conditions needed to be met before such a joint effort could be launched.
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.
The opposition Syrian National Council, meeting in Qatar ahead of weekend talks with Arab countries, called Russia's credibility "badly damaged."
Rights groups estimate that more than 6,000 people have died in the crackdown since mid-March.