Three Iranian pilgrims were killed and 15 were injured in a suicide attack on their bus traveling 50 kilometres south of Baghdad.
The bus was carrying pilgrims to the holy city of Karbala when the attack occurred, according to the Iraqi interior ministry.
The incident comes as tens of thousands of Shias are traveling to Karbala to mark the birthday of Mehdi, the 12th Shia imam.
Iranian pilgrims have been victims of several similar attacks in recent years as conflicts have intensified between Sunnis and Shias in Iraq.