Russian air raids on several areas in the south of Idlib province left six civilians dead, the Syrian Observatory for Human Rights said.

Regime air raids killed three civilians in the same region, which is controlled by Syria's former Al-Qaeda affiliate Hayat Tahrir al-Sham, the Britain-based monitor added.

The bombardment wounded 20 other civilians in total, it said.

Regime forces have in recent days chipped away at the jihadist-dominated region of Idlib on the Turkish border after months of deadly bombardment.

The gains come despite a deal reached between rebel backer Turkey and regime ally Russia last year to set up a buffer zone around the region to protect its some three million inhabitants.

Since Wednesday, the Syrian military has seized the key town of Khan Sheikhun from jihadists and allied rebels and overran the countryside to the south, encircling a Turkish observation post in northern Hama.

On Saturday, pro-government fighters gathered north of Khan Sheikhun in an apparent bid to press on with the offensive.

After Khan Sheikhun, Maaret al-Numan is the next town on a key highway running across Idlib province that analysts say is coveted by Damascus.

Total government control of that road would allow it to connect the capital with second city Aleppo, fully retaken from opposition fighters in late 2016.

The war in Syria has killed more than 370,000 people and displaced millions since it started in 2011 with a brutal crackdown on anti-government protests.