Syrian troops backed by helicopter gunships on Friday (September 21st) clashed with opposition forces near the Hanano army barracks in Aleppo province, the Syrian Observatory for Human Rights reported.
Elsewhere in the province, fighting broke out between troops and opposition forces near the Meng military airport, the monitoring group said.
North-west of Damascus, the Observatory reported a massive explosion believed to have been a car bomb. Heavy gunfire was heard afterwards but there were no immediate reports of casualties.
In Damascus, the official news agency SANA said the army found a mass grave containing the bodies of 25 people shot execution-style in the Qadam district.
Protesters also took to the street in Damascus, Deraa, Hama, Aleppo and Hasaka following Friday prayers, AFP reported.
Security forces fired on demonstrators in Hama, arresting a number of people, while demonstrators were wounded in Aleppo.
Overall, at least 11 people were killed in violence on Friday, including a civilian killed in shelling in Rastan, Homs province, the news agency said.