Regime forces launched a new blitz on the Syrian city of Homs, killing more than 50 people, activists said, as the UN weighed a joint mission with the Arab League to end the violence.
Shelling erupted at daybreak, killing 53 civilians in the besieged central city and burning several bodies beyond recognition, the Syrian Observatory for Human Rights said.
Government troops trying to crush opponents of President Bashar al-Assad have killed at least 400 people in a relentless six-day onslaught on Homs, 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, added that residents of the district were hiding on ground floors as there were no underground shelters.
"When you venture outside, you can see craters every 10 metres," he said.
At least 83 people were killed across Syria on Thursday, said the Britain-based Observatory.
Eleven died when a shell slammed into their house in the Homs neighbourhood of Inshaat, while six others were killed in Rastan, a restive town in the same province.
Elsewhere, rebels killed seven security forces members when they ambushed two buses at a bridge near the southern town of Daraa, 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 in Daraa.
UN chief Ban Ki-moon said on Wednesday the "appalling brutality" of the assault on Homs "is a grim harbinger of worse to come."
He launched the idea of sending a joint observer mission with the Arab League but the idea had 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, which would include a UN envoy.
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 the regime in 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 relentless 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, which was meeting in Qatar ahead of weekend talks with Arab countries, said Russia's credibility was "badly damaged" by its stance.
"Russia needs to restore its credibility in the eyes of the Syrian people by using its influence on the Assad regime" to immediately stop the killings and negotiate Assad's departure, it said.
Council spokesman Bassma Kodmani said: "Russia needs to get on the right side of history now and turn the page of the Assad regime together with the Syrian people."
Rights groups estimate that more than 6,000 people have died in the crackdown since mid-March.
Human Rights Watch urged the Syrian government to stop shelling residential areas of Homs.
"Those responsible for such horrific attacks will have to answer for them," HRW's Anna Neistat said in a statement.
Russia said it took a cautious view of US-backed attempts to forge a "friends of Syria" coalition that may soon meet in Turkey to coordinate humanitarian assistance for the embattled opposition.