Officials in northern Nigeria now say about 50 people were killed Sunday in the bombing of three Christian churches and subsequent rioting in Kaduna state.

The suicide bombings in the cities of Zaria and Kaduna killed at least 21 people. Christian youths later took the streets in Kaduna, setting fire to mosques and shops and attacking cars.

The updated death toll came Monday from emergency officials speaking on condition of anonymity.

The Islamist militant group known as Boko Haram has claimed responsibility for the attacks on the churches.

An e-mail sent by the group's purported spokesman, Abul Qaqa, said the attacks were reprisal for the alleged destruction of mosques and killing of Muslims by Christians.

Boko Haram has also claimed responsibility for church attacks the previous two Sundays. Those attacks killed 18 people.

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

Nigeria has endured recurring bouts of deadly violence between Muslims and Christians, most often in and around the city of Jos.

Nigeria's population of 150 million is split about evenly between the two religious groups.