A riot police training academy in Gwoza, Borno state has been overrun by Boko Haram Islamist militants, a witness in Borno has told the BBC.
Shots were heard after the militants arrived in three armoured vehicles and on dozens of motorcycles, he said.
A police spokesman confirmed the attack while a senior security source said it had not been possible to communicate with the academy since Wednesday.
The Liman Kara college is near Gwoza town, seized by Boko Haram this month.
The resident of Liman Kara, which is about 15km from Gwoza, told the BBC Hausa service that police recruits were seen running from the college after the attack began at dawn on Wednesday.
He said he was unable to confirm if there were casualties as he had joined other residents and fled the town to nearby hills.
A security official who did not want to be named told the BBC that the militants had “entered the school” but said he could not confirm they were in control of the college as it had not been possible to contact it.
A similar attack on the college was repelled by officers undergoing training there two weeks ago.
BBC Hausa’s Mahmud Lalo says the Liman Kara academy is one of only two riot police training colleges in Nigeria and the militants are likely to find weapons there.
According to the Nigerian online magazine, Sahara Reporters, several hundred militants were involved in the raid on the college, which there were reportedly more than 290 police trainees at the time.