Twelve civilians were killed Friday in a bomb blast on a bus at the northern edge of the Iraqi Shiite holy city of Karbala, medical officials said.

"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.

The officials said all of those killed were civilians who died in the explosion and subsequent fire in the bus. They spoke on condition of anonymity in line with regulations.

There was no immediate claim of responsibility.

Iraq declared victory over the Daesh terrorist group in 2017, but the group's sleeper cells continue to wage an insurgency and carry out sporadic attacks across the country.