The Syrian Observatory for Human Rights said the toll from the bomb blast, which occurred near a mosque in the town of Dmeir, was expected to rise because of the number of people seriously wounded.
The Britain-based monitor said a woman and a child were among those killed in the government air strikes on the Marj region elsewhere in Eastern Ghouta, a key rebel bastion outside Damascus.
Regime forces have besieged Eastern Ghouta for more than 18 months, carrying out daily bombardment in a bid to dislodge rebels.