Explosions have rocked three churches in northern Nigeria, where the radical Islamic sect Boko Haram has claimed responsibility for similar attacks.

Authorities say all three blasts happened on Sunday in Kaduna state, which divides Nigeria's mostly Muslim north and predominantly Christian south.

Two blasts hit churches in the city of Zaria, according to local officials. A third explosion took place at a church in Kaduna city.

Witnesses say there were casualties, but it was not immediately clear how many.

Investigators have not said who they believe carried out the attacks.

Boko Haram has claimed responsibility for attacks on central and northern churches the previous two Sundays. Those attacks killed 18 people.

The militant group has claimed it is fighting to establish a strict Islamic state throughout Nigeria. It does not recognize the government or the country's constitution.

The country of 150 million is about evenly divided between Muslims and Christians.