Dozens more were wounded in the raids on Deir Al-Assafir, a town in the opposition stronghold of Eastern Ghouta, the Syrian Observatory for Human Rights said.
The toll, which included two children and a civil defence volunteer, represented "the largest ceasefire violation in Eastern Ghouta," Observatory head Rami Abdel Rahman said.
A fragile ceasefire brokered by the United States and Russia has been in place across parts of Syria, including Eastern Ghouta, since February 27.
Eastern Ghouta is dominated by the powerful Jaish al-Islam (Army of Islam) rebel group, which has agreed to the truce, although Al-Nusra is also present.
The ceasefire has largely been hailed as a success by the United Nations, which says the lull in fighting has allowed the pace of humanitarian aid deliveries to besieged areas to pick up.
More than 270,000 people have been killed since Syria's conflict erupted in March 2011.