Another missile struck nearby as residents rushed to carry the wounded from the bus, the British-based Syrian Observatory for Human Rights said.
Syrian forces have killed more than 500 people, mainly civilians, since December 15 in a bombing campaign over Aleppo. It has been one of the most intense attacks on residential areas since the uprising against president Bashar Assad began three years ago.
Rebels seized parts of Aleppo, Syria's largest city, during the summer of 2012. Rebels also have fired at neighbourhoods perceived as loyal to Assad in other parts of Syria.
Yesterday, Syrian state media said a rebel-fired mortar shell killed three people in Homs. The Observatory said one person was killed in the same incident, but such conflicting claims are not unusual.