Hundreds of civilians have fled villages near the front line in Syria’s Idlib province fearing an imminent regime assault on the country’s last major rebel bastion, a monitor said Thursday.

Russia-backed government forces have been massing around the northwestern province for weeks, while on Tuesday Moscow carried out its first air strikes on the rebels in three weeks.

Late on Wednesday, hundreds of villagers abandoned areas in the southeast of the province that lie close to government-held territory, the Syrian Observatory for Human Rights said.