The Syrian army said it had seized mountainous territory from Islamic State to the east of the road linking Damascus to Aleppo, helping to secure a critical lifeline for the government that has often come under IS attack.

The army also said it had seized 22 villages and farms from IS near Maskaneh, the last major IS-held town in Aleppo province. The capture of Maskaneh will bring Russian-backed government forces to the border of Raqqa province, much of which is held by US-backed groups that are also fighting IS.

Government forces supported by Iranian-backed militias and the Russian army have stepped up attacks on Islamic State on several fronts in recent weeks as a "de-escalation" deal brokered by Russia and Turkey has reduced fighting in western Syria.

The army said it had captured the northeastern and central portions of a mountain range to the east of the Khanaser-Ithriya road. "It certainly widens the circle of security around the Damascus-Aleppo road," a military source told Reuters.

The Khanaser-Ithriya road is the only government-controlled route linking Aleppo to other government-held cities of western Syria. Sections of the main highway to Aleppo, which runs through western Syria, remain in rebel hands.

The town of Maskaneh is located on the western banks of the River Euphrates in Aleppo province some 10 km from the provincial border with Raqqa. The next major urban centre to the east is Tabqa, which was captured from IS by the U.S.-backed Syrian Democratic Forces (SDF) in May.

The United States has so far ruled out cooperating with the Russian-backed government in the fight against IS in Syria, where the group has been forced into retreat by the separate campaigns being waged against it.