Syrian troops on Friday reclaimed a cluster of towns they had lost early in the eight-year-old war, pressing their offensive in the northwest, the country's last big rebel stronghold.

Syrian rebels withdrew from a town in southern Idlib province and from their last remaining territory in neighbouring Hama province after government troops advances in the area

At least 15 people were reported killed on Sunday in shelling by government and insurgent forces in northwestern Syria, further straining a Russian-Turkish ceasefire deal for the region.

Syrian rebels denied on Sunday they had pulled any heavy arms from a major opposition bastion in the north, as the deadline to implement a demilitarisation deal there draws closer.

The Syrian army and its allies made advances in the southwest that bring it closer to the Israeli-occupied Golan Heights border, state television and rebels said on Saturday.

Syrian rebels have agreed with government backer Russia to the negotiated surrender of a sensitive southwestern zone bordering the Israeli-annexed Golan Heights, a monitor said Thursday.

The evacuation of thousands of people from two loyalist Syrian villages that were besieged by rebels in the northwest is complete, state media said on Thursday, with the government expected to release hundreds of detainees in return.

Unidentified air strikes have killed 12 civilians in a rebel-held pocket of southern Syria as regime ally Russia presses talks for Damascus to retake the area, a monitor said Wednesday.

Air strikes on Tuesday killed seven civilians in a sensitive southwestern province of Syria bordering the Israeli-occupied Golan Heights, a war monitor said.

A string of Syrian rebel-held towns and villages accepted government rule on Saturday as insurgent lines collapsed in parts of the southwest under an intense bombardment that the United Nations says has forced 160,000 people to flee.

Syrian rebels are negotiating with Russia to return state sovereignty in areas they control in the southwestern Deraa province, where the army has made rapid advances this month, insurgents said on Saturday.

Syrian government forces and their Iran-backed allies will face "volcanoes of fire" if they launch a threatened offensive in the opposition-held southwest, a rebel commander told Reuters on Tuesday.