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 opposition bastion, AFP reported Thursday (September 6th).

Russia-backed government forces have been massing around the northwestern province for weeks, while on Tuesday Moscow carried out its first airstrikes on the opposition 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.

They were heading north towards opposition-held territory in neighbouring Aleppo province, the monitor said.

"Around 180 families, or some 1,000 people" have hit the roads since late Wednesday, Observatory chief Rami Abdel Rahman said.

Meanwhile, rescue workers in Idlib said Thursday they were forced to halt operations after airstrikes damaged their centre.

The White Helmets rescue force said a salvo of rockets and shells hit their headquarters in al-Tamana in a southern part of Idlib on Thursday morning, followed by airstrikes.

Mossab al-Qadur, the head of the centre, told AFP he was inside the building when it was struck by dozens of shells and rockets.

"When the shells stopped, we quickly got out. About 15 minutes later, a Sukhoi 24 (Russian jet) targeted the centre, which put it out of service," said Qadur.

"We had gotten the equipment out before the airplane targeted it, and we are all okay," he said.

Humanitarian groups and the UN have warned that any offensive on Idlib would risk a "humanitarian catastrophe".

On Thursday, French Foreign Minister Jean-Yves Le Drian warned that there was a danger of a "humanitarian deflagration" in Idlib.

"The regime of Bashar al-Assad wants to take Idlib militarily, and we are warning everyone involved of the risks of humanitarian deflagration that could result," he said.