Damascus -The Syrian opposition's new leader said on Sunday that President Bashar al-Assad's regime was on its "last legs," even as Russia warned it would block any move at the UN to use force against its ally.

Western governments launched a push for tough new sanctions against Damascus amid an outcry over a series of massacres blamed by UN observers on pro-regime forces as troops killed at least 83 civilians on Saturday, a human rights watchdog said.

"We are entering a sensitive phase. The regime is on its last legs," Kurdish activist Abdel Basset Sayda told AFP shortly after he was named as the new leader of the opposition Syrian National Council.

"The multiplying massacres and shellings show that it is struggling," he added in allusion to a spate of mass killings of civilians, the most recent of which saw 20 people, most of them women and children, killed in the bombardment of a residential area of the southern city of Daraa on Saturday.