The Islamist rebels, including fighters from Central Asia and Chechens, as well as jihadists from Syria's Al-Qaeda branch, were pressing an advance on the village of Jureen, Rami Abdel Rahman, director of the Syrian Observatory for Human Rights, told AFP.
"There has been heavy fighting in the village of Bahsa, less than two kilometres (1.2 miles) from Jureen," he said.
Perched on a plateau in the central province of Hama, Jureen stands between Sahl al-Ghab, a plain where Assad's army has for several days been fighting the rebels, and the pro-Assad Latakia province to the west.
If the rebels manage to capture Jureen, they will be able to advance into the mountains of Latakia and bomb several communities from Assad's Alawite sect, an offshoot of Shiite Islam.
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Among them would be Qardaha, Assad's ancestral town and home to the tomb of his father and predecessor Hafez al-Assad.
"It will then be an existential battle for the Alawites," Abdel Rahman said. "There are calls for Alawite youth to take up arms and to defend the areas surrounding Jureen."
Since the end of March, the Syrian army has suffered a series of setbacks in the northwest of the country, with the rebels repelling them from almost all of the Idlib province to the north of Sahl al-Ghab.
The rebels have advanced south since the end of July and launched an offensive in Sahl al-Ghab.
Assad's regime has been at war against different rebels groups for the last four years, in a conflict that has so far killed at least 230,000.