The blast aboard a bus at a checkpoint north of Karbala also wounded five people, according to the city's health authorities.

Security forces said Saturday they had arrested a man suspected of placing the explosives on the bus before it disembarked.

Iraq declared victory against IS in late 2017 after three years of a brutal fightback against the extremist Sunni group, which had specifically targeted Shiite gatherings.

Jihadist sleeper cells have continued to carry out hit-and-run attacks against government positions across the country, particularly at checkpoints, but attacks targeting Shiite religious gatherings had been rare in recent years.

The deadliest incident this year was a stampede earlier this month in Karbala that left more than 30 pilgrims dead and dozens injured.

Hundreds of thousands of pilgrims from Iraq, Iran and as far as India had been gathering in the southern city this month 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.