(Adds details, background)
LAGOS, June 17 (Reuters) - Two explosions shook the town of Zaria in northern Nigeria's Kaduna state on Sunday, the emergency services said, and at least one of them was at a church, a security official said.
Islamist sect Boko Haram has often attacked church services in Nigeria, split roughly evenly between Christians and Muslims.
Boko Haram, which has become increasingly radicalised and meshed with other Islamist groups in the region, including al Qaeda's north African wing, is the leading security threat to Nigeria, Africa's top oil producer and a member of OPEC.
Regular attacks on Sunday church services are usually claimed by the sect, which says it is fighting to reinstate an ancient Islamic caliphate that would adhere to strict sharia (Islamic law).
Islamist militants attacked two churches in Nigeria last Sunday, spraying the congregation of one with bullets, killing at least one person, and blowing up a car in a suicide bombing at the other, wounding 41.
The Islamists' leader, Abubakar Shekau, has justified attacks on Christians as revenge for killings of Muslims in Nigeria's volatile "Middle Belt", where the largely Christian south and mostly Muslim north meet.
Kaduna is close to the Middle Belt areas. (Reporting by Tim Cocks; Editing by Janet Lawrence)