"Twelve civilians have been killed and five wounded in an explosion on a bus at the main checkpoint for the northern entrance to Karbala", the city's health authorities said, adding the victims included women and children.

Security forces arrived quickly to the scene of the attack, which was not immediately claimed.

Attacks have been rare in Karbala in recent years, but sleeper cells of the Islamic State group, which Iraq declared victory against in late 2017, continue to carry out attacks across the country.

On September 10, more than 30 pilgrims were killed and dozens injured in a stampede at a major shrine in Karbala as hundreds of thousands flocked to the city to mark the Shiite holy day of Ashura.

Iraq is expecting millions more Shiite pilgrims to arrive at the end of October for the annual Arbaeen commemoration, which marks the end of the 40-day mourning period for the seventh-century killing of Imam Hussein by the forces of the Caliph Yazid.