An attack on the Syrian city of Homs which the opposition says killed more than 200 civilians is a "further step in savagery" and a "crime against humanity" by the Damascus regime, France said Saturday.

And in a swipe at Syria's ally Russia, French Foreign Minister Alain Juppe warned against blocking a UN Security Council resolution aimed at ending the violence that has left more than 6,000 people dead.

"I condemn in the strongest terms the new attack by the Syrian regime on the people of Homs," Juppe said in a statement.

"Far from halting their policy of repression, the Syrian authorities have taken a further step in savagery. The massacre of Homs is a crime against humanity, its perpetrators must answer for it.

"This unbridled violence underlines the urgency for the United Nations Security Council to end its silence in order to condemn the authors of this crime and open the way to the implementation of the Arab League's political plan.

"The international community must recognise and support the right of the Syrian people to freedom, security and the choice of their political future.

"Those who would hinder the adoption of such a resolution would assume a heavy responsibility in history."

Opposition groups Saturday put the death toll in Homs at between 217 and at least 260. If confirmed, the violence would be the deadliest in the 10-month uprising against the regime of defiant President Bashar al-Assad.

The Syrian government denied involvement in the early morning assault, blaming groups trying to incite unrest ahead of a UN vote, as television images showed bodies and buildings destroyed in a city turned into a war zone.

A diplomat at the United Nations said the Security Council was expected to vote on a resolution of condemnation later Saturday.

But there were new objections from Russia, which opposes a resolution that can be used to justify foreign military intervention, call for Assad to quit or impose an arms embargo on Syria.