At least seven people have been killed and dozens injured in a suicide bombing during mass at a Catholic church in northern Nigeria, officials say. The attack happened in Kaduna, scene of many sectarian attacks. The blast was caused by at least one vehicle being driven towards the church loaded with explosives, witnesses said. The building was badly damaged. Reports said at least one person was killed in reprisal attacks by Christians after the bombing. No group has said it carried out the bombing, but Islamist group Boko Haram has been involved in several similar attacks in the area. A spokesman for the local governor has called for calm. Speaking on local radio, he pleaded with people not to retaliate. Looking for sanctuary The attack happened at St Rita's church in the Malali neighbourhood of the city. Reports say a car drove up to the gate of the church, was refused entry and detonated its explosives outside the church. The church was surrounded by soldiers and police after the blast, and ambulances were taking the injured to hospital. Nigeria is roughly divided between a largely Muslim north, and the south, where Christianity and traditional religions dominate. Kaduna is on the dividing line between the two areas. The BBC's Will Ross in Kaduna says many people have come to the city in recent months in search of sanctuary from violence in other parts of northern Nigeria. Boko Haram is fighting to overthrow the government and impose Sharia law, and has admitted carrying out previous attacks on churches. In June it attacked three churches in the state of Kaduna, including one in the city. At least 50 people were killed in the bombings and the reprisals that followed.