Northern Nigeria's riot police training academy has been overrun by Boko Haram Islamist militants, a witness in Borno state 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.

Thousands have been killed across north-eastern Nigeria since Boko Haram launched its violent campaign for an Islamic state in 2009.

The militants have stepped up their attacks after being pushed out of their bases in Maiduguri, the capital of Borno state, and have been targeting towns and villages in deadly raids.

In recent weeks, the militants have been moving from their rural camps and taking over substantial towns.

'Residents fled'

The militants have been in control of Gwoza, which had a population of about 50,000, since the beginning of August.

They apparently retreated about 100km (62 miles) to Gwoza after losing control of Damboa - both large towns in Borno state.

But attempts by the security forces to retake Gwoza have failed - and a group of about 40 soldiers is now refusing to fight, saying they are too poorly equipped to take on the heavily armed insurgents.

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 newspaper 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.

In April, Boko Haram caused global outrage by abducting more than 200 girls from a boarding school in the remote town of Chibok in Borno state.

The US, UK, France, China and Israel have all offered assistance in helping track them down, but their whereabouts remains a mystery and the militants' attacks have increased.