A government offensive in the north would be the latest push by Assad's forces to recapture territory ahead of peace talks planned for January in Geneva. The opposition currently holds large swathes of territory in the north, including along the border with Turkey, as well as whole districts of Aleppo, Syria's commercial center.
The Tawhid Brigade is one of Syria's best known and powerful rebel groups, with an estimated 10,000 fighters. It's particularly strong in Aleppo province.
Assad's troops have also been battling opposition fighters in central Syria's rugged Qalamoun region near the Lebanese border in order to cut off rebel supply routes and stem the flow of fighters from its neighbor.
On Sunday, the Observatory said troops fought rebels, including members of the al-Qaida-linked Jabhat al-Nusra, inside the predominantly Christian town of Maaloula near Damascus. Heavy clashes were concentrated in the town's old quarter and there were casualties on both sides, Abdurrahman said. He could not provide the number of those killed and wounded in the fighting.
Maaloula lies on the edge of Qalamoun, about 60 kilometers (40 miles) northeast of the capital. The town had previously been firmly in the government's grip despite being surrounded by rebel-held territory. It's just southwest of Nabek, a town that has been the focus of a government push for the past three days. Troops already captured two other major towns in the area.
Maaloula was a major tourist attraction before the civil war. Some of its residents still speak a version of Aramaic, a biblical language believed to have been used by Jesus.