A strong explosion shook the Shiite holy city of Karbala (search) on Monday, forcing police to cordon off the area, witnesses said.

There was no immediate word on casualties.

The blast came a day after a car bomb exploded at Karbala's main bus station, killing 13 people and wounding 33. Another bomb, in nearby Najaf (search), killed 54 people and wounded nearly 200.

Witnesses said Monday's explosion occurred in the city's Saadiya neighborhood, adding that security forces had sealed off the area.