A missile struck a bus Tuesday in a rebel-held area of the northern Syrian city of Aleppo, setting it on fire and killing at least 10 people, activists said.

Another missile stuck nearby in the al-Bab area of Aleppo as residents rushed to carry out 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 Dec. 15 in a bombing campaign of Aleppo, mostly by dropping crude explosive-filled barrels over rebel neighborhoods. The al-Bab area has been a frequent target. The bombing has been one of the most intense attacks on residential areas since the Syrian uprising began three years ago against the rule of President Bashar Assad.