Three people survived the attack but all are seriously wounded in hospital. Others travelling on the bus were unharmed.
It is believed the shooting took place as a revenge killing for the stoning to death of a Yazidi woman last week.
The female had reportedly fallen in love with a Muslim man and converted to Islam but was beaten to death by a group of Yazidi males with stones, sticks and lumps of concrete.
The ancient pre-Islamic Yazidi religion, whose origins are unknown, has around half a million adherents around the world. Most of these are located in the northern parts of Iraq but they prefer to play down their links to the ethnic Kurds who dominate the area.
The Yazidi faith worships an angel often associated by both Muslims and Christians as being the devil.
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