Syrian regime forces advanced against militants on Saturday on the edge of the northwestern province of Idlib, the last outside government control, a monitor and the state news agency said.

Government and allied forces backed by Russian warplanes have since Monday been fighting mostly militants in an area straddling the border between Idlib and Hama provinces.

The fighting, which could signal the start of a major offensive to seize Idlib province from rebels dominated by a former Al Qaeda affiliate, escalated on Thursday.

On Saturday, regime troops advanced, seizing several villages and surrounding areas, the Syrian Observatory for Human Rights, a Britain-based war monitor, said.

State news agency SANA said regime forces had taken control of five villages, areas and “strategic hills” in the northeast of Hama province.

An AFP correspondent said civilians continued to flee areas near the battle zone, heading north towards the provincial capital of Idlib with their belongings piled high on pick-up trucks.

Since Thursday, the clashes have killed 32 soldiers and allied members of paramilitary units, as well as 29 rebels from Islamist groups or from former Al Qaeda affiliate Fateh al-Sham, the Observatory said.

Twenty-one civilians have also died.

Government forces' first aim to take control of the southeast of Idlib province, Observatory head Rami Abdel Rahman said.

This would allow the regime to have full control of a road that links the capital Damascus to the government-held second city of Aleppo.

Russia intervened on the side of President Bashar al-Assad's regime in 2015, helping it to take back control of swathes of territory including Aleppo.

The war has killed more than 340,000 people and displaced millions from their homes since it started in 2011 with the brutal repression of anti-government protests.