Syria is heading for a catastrophic partition and could see the return of Daesh if there is no inclusive peace settlement, UN Syria envoy said.

Staffan de Mistura on Monday told an audience at Geneva's Graduate Institute that if the Sunni Muslims who account for the majority of the population will not be included in the political process, "Daesh will come back".

"Neither the European Union nor the World Bank would help fund the estimated $352 billion cost of rebuilding Syria unless there was a political process with a new constitution, elections under UN supervision and power-sharing. Without that any military victory would be a Pyrrhic victory, UN Syria envoy said.

Syrian government forces have splintered eastern Ghouta into three separate zones in a month-long assault to crush the last major opposition stronghold near Damascus, confronting the rebels with their biggest defeat since 2016. 

On another front Turkish troops and their rebel allies swept into the northwest Syrian town of Afrin on Sunday, the culmination of an eight-week campaign to drive the Kurdish YPG fighters from the region.