The attack comes as Assad's forces went on the offensive earlier this month into the rugged Qalamoun region near the Lebanese border in order to cut off rebel supply routes and stem the flow of fighters.
Maaloula is on the edge of Qalamoun, southwest of the town of Nabek that has been the focus of a government push for the past three days. Troops already captured two other major towns in the area.
"The aim of this operation is to cut regime supply lines from Damascus to Nabek," said an activist who goes with the name of Khalil al-Harastani. He is based in Qalamoun.
The Observatory and al-Harastani reported heavy fighting in Nabek on Saturday, saying the army's air force was taking part in the government offensive.
Maaloula overlooks the highway that links Damascus with the central city of Homs, Syria's third largest. The highway has been cut since mid-November when the fighting in Qalamoun began.
A Maaloula resident told The Associated Press in Damascus that the rebels are now in the western part of the town and have captured the Mar Takla convent, where several nuns were staying.
Earlier Saturday, Syrian troops shelled a marketplace near the northern city of Aleppo, killing at least six in what appeared to preparations for a government push to clear the northern city of rebels, activists said.
The shelling hit the Nafasin market in the rebel-held town of al-Bab, said Rami Abdurrahman, who heads the Observatory.
An Aleppo-based activist who goes by the name of Abu Raed corroborated the report, saying said the strike was likely to attack fighters who filter through the town into rebel-held areas of eastern Aleppo. Abu Raed, like al-Harastani in Qalamoun, does not use his real name for fear of government reprisals.
Assad's forces and rebels seeking his overthrow are poised for battle in Aleppo, Syria's largest city and former economic capital, much of which has been reduced to rubble.
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Mroue reported from Beirut. Associated Press writer Diaa Hadid contributed to this report from Beirut.