Three blasts have hit churches in the northern Nigerian state of Kaduna. Local emergency services reportedly rushed to the scene but did not release any reports of casualties. All of the blasts happened close to the city of Zaria. Kaduna state has previously seen attacks by the radical Islamist group Boko Haram. Last week the group attacked two church services, sparking violence which killed seven people. Hundreds have died in its previous attacks on churches. Nigeria's National Emergency Management Agency (Nema) said the blasts happened in the Wusasa and Sabon-Gari districts of Zaria. Wusasa is the site of the first church to be built in northern Nigeria. Boko Haram has previously justified attacks on churches by saying they were carried out in revenge for killings of Muslims in central Nigeria during earlier bouts of violence. Recently, hardly a Sunday goes by without reports of churches being attacked in Nigeria, the BBC's Will Ross reports from Lagos. 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, our correspondent says. Last weekend an archbishop in central Nigeria appealed to Christians not to retaliate when churches are attacked, he adds.