Gunshots and an explosion have been heard at a teacher training college in the northern Nigerian city of Kano.
Students were seen fleeing from the city's Federal College of Education.
One student told the BBC by phone that he had seen 17 dead bodies at the scene.
It is not clear who was responsible, although suspicion will fall on militant Islamist group Boko Haram, which has been waging an insurgency in Nigeria since 2009.
"Our men are already there. I've called them and they've said the crowd is too big. We are yet to establish the exact number who were injured or killed," a military spokesman in Kano told Reuters.
In July the city suffered a spate of five attacks in four days, one of which also targeted a college and killed six people.
In May 2013, Nigeria's President Goodluck Jonathan imposed a state of emergency in the northern states of Borno, Yobe and Adamawa, vowing to crush the insurgency.
However the militants have stepped up attacks, killing more than 2,000 civilians this year, according to New York-based Human Rights Watch.
Boko Haram's name translates as "Western education is forbidden", and it has carried out several attacks on schools and colleges, seeing them as a symbol of Western culture.
In April, it raided a boarding school in Chibok town in the northern state of Borno, and is holding more than 200 girls that its gunmen abducted during that attack.