ISLAMABAD — A senior Pakistani police official says a bomb blast at a mosque belonging to the minority Shiite sect of Islam has killed six people.

Abdul Qudoos Kalwar, a senior police official in Shikarpur where the blast took place, says four of the dead are children.

Shikarpur is in Sindh province, roughly 500 kilometers (310 miles) north of the port city of Karachi. Kalwar says it is not yet known whether it was a bomb planted on the scene or a suicide attack.

There was no immediate claim of responsibility. Sunni Muslim extremists have often targeted religious institutions of Shiites, whom they do not consider to be true Muslims.