Story highlights The number of casualties is unclear

The gunmen targeted a lecture hall used by Christians for worship

The gunmen opened fire and threw four or five bombs, a journalist says

A university professor says he heard gunfire and multiple explosions

Gunmen attacked Christians worshipping on a Nigerian university campus on Sunday, with witnesses reporting multiple explosions and gunfire.

The number of casualties was not immediately clear.

Professor A.B. Baffa said he was at home on the campus of Bayero University in the city of Kano when he heard the gunfire and explosions. When he went to see what was going on, he said he saw people fleeing and saying gunmen attacked areas where Christians were worshipping.

The gunmen began their attack around 8:30 a.m., targeting a lecture hall normally used by Christians for Sunday services, journalist Salihu Tanko reported from the scene.

"Reports say that they came in one vehicle and they also came on the bike and started shooting sporadically and at the same time threw about four or five small locally made bombs," Tanko reported.

Baffa said police have closed off the area.

The attack happened while the university is on a break, so most of the students are not on campus, Tanko said.

Britain's Foreign Office said it had received reports of an explosion and gunfire at Bayero University and that the incident may be ongoing.