The city has been carved into opposition- and government-held areas.
Syria's civil war, now into its third year, has killed more than 120,000 people, according to activists.
Millions have fled their homes because of the fighting.
In central Syria, a suicide bomber blew himself up near a primary school in the predominantly Shiite town of Umm al-Amed in Homs province, killing at least 12 people, half of them children, the Observatory said.
Syria's state news agency put the death toll at 20, including six children. It wasn't immediately possible to reconcile the conflicting tolls.
The bombing highlighted the sectarian overtones of the Syrian civil war, which is now in its third year.