Soon after, a second blast killed at least two people and wounded 11, and a third blast left at least one dead and five wounded in Baghdad's al-Habibiya district.

All three blasts targeted buses full of Shia Muslims returning from pilgrimage at the tomb of Hussein, the Prophet Mohammed's grandson, who was killed in battle in Karbala in the year 680.

Security in Karbala had been tight for the pilgrimage to mark the 15th night of the Muslim month of Shaban.

The attacks on the Shia worshippers came amid increased sectarian violence between Iraq's Shia and Sunni Muslims.