The bomb, which exploded on the highway from Balad to Samarra, also wounded 22 people, most of them Iranians.

It followed another car bomb against pilgrims travelling the same route on Monday that killed eight people and wounded at least 15.

In both cases, the pilgrims were on their way to the holy shrine of Imam Hassan al-Askari (peace be upon him) in Samarra, which was bombed in February 2006, unleashing a wave of sectarian bloodletting in which tens of thousands of people died.

Iraq is home to some of the holiest sites in Islam and is visited by hundreds of thousands of foreign pilgrims each year, most of them from neighboring Iran.

Terrorists in Iraq, including those linked to al-Qaeda, frequently target pilgrims.