Syrian activists say a missile has struck a bus in a rebel-held area of the northern city of Aleppo, killing at least 10 people.

The British-based Syrian Observatory for Human Rights said Tuesday that another missile landed nearby as medics were evacuating the wounded from the first explosion.

The Observatory says the missiles were fired from a plane and that at least two children were among the killed. It gave no further details on the missiles used.

Syrian forces have killed over 500 people, mostly civilians, since Dec. 15, in an intensified bombing campaign of Aleppo, mostly dropping crude dynamite-filled barrels over rebel neighborhoods.

The fighting has divided the city into rebel-held areas and those loyal to the government of President Bashar Assad.