Regime forces launched a new blitz on the Syrian city of Homs on Thursday, killing dozens of people, activists said, as the UN weighed a joint mission with the Arab League to end the unrest.

Activists said at least 24 people were killed in the besieged central city as the shelling resumed at dawn, adding some of them were burned beyond recognition.

At least 400 people have died in Homs in a relentless onslaught by government troops that began very early on Saturday, activists say.

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

Omar Shaker, an activist in Baba Amr also reached by phone, added 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 (yards)," he said.

In eastern Deir Ezzor province, machinegun fire wounded dozens of people including women and children in Koriyeh, the Syrian Observatory for Human Rights said, adding that army reinforcements were being sent into the town.

UN chief Ban Ki-moon said 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 as he bemoaned the UN Security Council's failure to agree a resolution on the crisis.

The pan-Arab bloc's secretary general, Nabil al-Arabi, said he had spoken with Ban on the proposed mission, which would include a UN envoy.

The 22-member League suspended its month-long monitoring mission to Syria on January 28 because of the mounting violence.

The UN secretary general said consultations would be held with the Arab League and UN Security Council members in coming days "before fleshing out the details."

Ban hit out at Russia and China for their steadfast refusal to back UN resolutions condemning the violence in Syria, saying their move had encouraged the regime of President Bashar al-Assad to continue with the repression.

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

Russian Prime Minister Vladimir Putin said Wednesday that any outside intervention would have the destructive effect of "a bull in a china shop."

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

"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.

The opposition Syrian National Council 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 more than 6,000 people have died in the crackdown since mid-March.

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

But British Prime Minister David Cameron said he had little confidence in such promises and urged the Russians to "look at their conscience and realise what they have done."

Germany said it was expelling four diplomats from the Syrian embassy in Berlin after the arrest of two men suspected of spying on regime opponents.

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

"We are determined to establish a broad-based forum to promote international understanding with all countries concerned," Foreign Minister Ahmet Davutoglu said.

The conference could take place in Turkey or in another country but it must certainly be "in the region" and "as soon as possible", he added.

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