At least 15 people were killed and 30 others injured when gunmen attacked a college in the northeastern Nigerian state of Kano, a police official has said.
Kano State police commissioner, Adelere Shinaba, said on Wednesday that the gunmen, whom he described as "insurgents", ran onto the grounds of the Federal College of Education after exchanging fire with police outside.
Al Jazeera's Ahmed Idris, reporting from Abuja, said gunmen stormed the building and opened fire on students. There were also reports of a suicide bombing, our correspondent said.
There was no immediate claim of responsibility for the attack.
"This attack comes after a period of quiet in Kano, and no doubt, fingers will be pointed at Boko Haram," Idris said. "This
new development will raise fears that they have larger sleeper cells in Kano than first thought."
Most of the victims at the northern teacher training college were in a lecture hall, where the two gunmen ran and opened fire on students.
One student who was having lunch nearby and asked not to be identified, said he saw the gunmen, who were dressed in black, and heard them shouting for all female students to lie face down.
There had been no bombings for more than a month during which Boko Haram has focused on taking a string of towns and villages along the northeast border with Cameroon where it has declared an Islamic caliphate and is enforcing a strict version of Islamic law.
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The group is threatening to attack Maiduguri, the Borno state capital and birthplace of its movement.
The military claimed to have killed about 100 fighters in a battle at the town of Konduga, 45km from Maiduguri last week.
It was a rare victory for the Nigerian military, which has failed to contain the insurgency.
Boko Haram attracted international attention with the kidnappings of more than 270 schoolgirls in April. About 50 escaped on their own.
The government and soldiers have failed to rescue any of the others, saying any military campaign could endanger their lives.
President Goodluck Jonathan has refused to consider a prisoner swap, with Boko Haram demanding the release of hundreds of detained fighters in exchange for the girls.
Source: Al Jazeera and agencies