At least 15 people have been killed in a gun and bomb attack at a university in Nigeria's northern city of Kano, witnesses and police said. Bodies were lying around Bayero University campus where Christian worshippers were holding a service. A bomb squad and military units are searching for the gunmen. No group has said it launched the attack, but the violent Islamist Boko Haram group is active in Kano. It has recently attacked churches. Nigeria's central government has struggled to contain the militant group, which operates mainly in the predominantly Muslim north, but has also struck as far south as the capital, Abuja. Sunday's attack took place in one of the lecture theatres used as a place of worship by Christians. A witness told AFP news agency the attackers had first thrown in explosives and fired shots, "causing a stampede among worshippers". "They now pursued them, shooting them with guns. They also attacked another service at the sporting complex." Another witness spoke of "pandemonium", and said he had seen two men shooting indiscriminately. Mohammed Suleiman, a history lecturer at the university, said security guards had to run for their lives when the violence broke out. "For over 30 minutes a series of bomb explosions and gun shots took over the old campus, around the academic blocks," he told Reuters news agency. Officials have confirmed seven deaths so far, but warn that up to 20 were killed as bodies were lying around and being taken to hospitals. "I counted at least 15 dead bodies. I think they were being taken to the Amino Kano teaching hospital," a witness told the agency. He added that he had seen many more people being treated for injuries. Security officials and doctors at local hospitals also say at least 15 people were killed. Kano state police spokesman Ibrahim Idris said that by the time police arrived, the attackers had "disappeared into the neighbourhood". A manhunt is under way. Boko Haram carried out a bombing in Kano in January that killed more than 180 people, its deadliest attack to date.