At least six people have been killed in a gun and bomb attack at a university in Nigeria's northern city of Kano, witnesses and police said.
A bomb squad and military units are searching for the gunmen who mounted the attack at Bayero University.
Reports suggest the violence may have targeted Christian students who were holding a religious service in one of the lecture theatres.
An army spokesman said several wounded people had been taken to hospital.
"The attack took place in one of the lecture theatres used as a place of worship by Christians," said Lt Iweha Ikedichi.
"There were casualties, but I don't have exact figures. Some have been rushed to local hospitals."
He said soldiers and police had surrounded the university and were searching for the gunmen.
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.
A BBC Hausa service reporter had seen six bodies at the university, and local police said the death toll could be higher.
No group has said it launched the attack, but the Islamist militant Boko Haram group is active in Kano.
Boko Haram carried out a bombing in Kano in January that killed about 150 people, its deadliest attack to date.
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.