A radical Islamist sect on Monday claimed responsibility for suicide bomb attacks that left at least 36 people dead in northern Nigeria on Sunday.The group, known as Boko Haram, made the announcement in an e-mail statement received by a news agency on Monday.Kaduna state authorities have imposed a 24-hour curfew as soldiers and police make efforts to restore order.Suicide bombers targeted four churches in a series of attacks that also prompted reprisal attacks by Christian youth against local Muslims. In all, two churches were attacked in Kaduna city while another two were bombed in a nearby Zaria city.In Zaria town, two churches were bombed as a bomber drove into Christ the King Catholic Church (CKC), Sabon Gari, and a similar attack took place at the Evangelical Church Winning All (ECWA), Wusasa.Red Cross officials in the city said in a church located in Zaria town, most of those killed or wounded were children.Kaduna state has previously seen attacks by Boko Haram.Last Sunday also, the group attacked two church services, sparking violence which killed seven people. Hundreds have died in its previous attacks on churches.Boko Haram says it wants Islamic sharia law in place across Nigeria and analysts suggest it is trying to trigger clashes between Christians and Muslims.A country of 150 million people, Nigeria's population is equally divided between Christians and Muslims.