ISIS on Saturday claimed a bomb blast that killed 12 people near the Iraqi Shiite city of Karbala the previous day.
The blast on a bus at a checkpoint north of Karbala also wounded five people, according to the city's health authorities.
Security forces said on Saturday they had arrested a man suspected of placing the explosives on the bus.
Prime Minister Adel Abdul-Mahdi in a statement released by his office Saturday gave no further details about the suspect.
Parliament Speaker Mohamad Al Halbousi expressed in a statement his frustration with the repeated "failures of intelligence agencies" to prevent such attacks. He added that security plans should be reviewed and intelligence gathering intensified.
On Saturday, security was tight on the roads entering Karbala with added checkpoints searching cars.
Iraq declared victory against ISIS in late 2017 after three years of a brutal fighting against the extremist group, which had specifically targeted Shiite gatherings. It was one of the biggest attacks since the group's defeat.
Extremist sleeper cells have continued to carry out hit-and-run attacks against government positions across the country, particularly at checkpoints, but attacks targeting Shiite 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 Ashura. It was the deadliest stampede in recent history during the commemorations.
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.
