A car bomb killed at least 30 people on Friday in the town of Rankus north of Damascus, the Syrian Observatory for Human Rights said.
Meanwhile, the Observatory said 11 people were killed in a regime air raid in northern Aleppo province.
"At least 30 people were killed and dozens more injured in a car bomb blast that targeted worshippers at the Khalid Bin Walid mosque in Rankus," the Observatory said.
It added that regime forces shelled the area afterwards.
Rankus, some 30 kilometres (18 miles) north of Damascus, is a Sunni town that backs the Sunni-dominated opposition to the government of President Bashar al-Assad.
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Local activists called on residents to avoid gathering for fear of additional attacks in the town, which has regularly been the scene of fighting between rebels and regime forces.
State news agency SANA reported the blast. Without elaborating, it said the explosion was the result of "a dispute between terrorist groups over the division of weapons and ammunition."
A video of the aftermath of the blast, filmed from a distance, showed a large cloud of black smoke and flames rising from a site next to a mosque.
In Aleppo province, the Observatory reported nine rebel fighters, a woman and a child were killed in a regime air raid on the village of Hadir.
More than 110,000 people have been killed since the beginning of the Syrian conflict in March 2011, according to the Observatory.
The violence, which began with peaceful protests against the regime of President Bashar al-Assad, has also displaced millions inside Syria and forced at least two million to seek refuge abroad.