Around 80 children who were held captive by Nigerian terror group Boko Haram do not remember their names or where they come from, an NGO has said.

The children, aged between five and 18, were freed from a camp in northern Cameroon last November. They do not speak English, French or any local language, according to US-based National Democratic Institute (NDI) director Christopher Fomunyoh, who visited the orphanage where the children are being rehabilitated.

Who are Boko Haram militants?

Boko Haram, which fights against Western influence in Nigeria and aims to impose its version of sharia law in the country, declared an Islamic caliphate in Gwoza, along the Cameroon border, in August 2014.

The group has been raiding several cities in the north of the country in a bid to take control of more land.

Violence linked to Boko Haram's insurgency has resulted in an estimated 10,000 deaths between 2002 and 2013.

Three states, Adamawa, Borno and Yobe, have been under a state of emergency since May 2013 due to Boko Haram's deadly attacks.

During an interview with the BBC, Fomunyoh explained that the children had spent so long with their captors that they had lost track of who they were.

"They've lost touch with their parents," he said. "They've lost touch with people in their villages, they're not able to articulate, to help trace their relationships, they can't even tell you what their names are."

Boko Haram terrorists are known for kidnapping children and using them in attacks they carry out throughout northern Nigeria, where thousands of people have been killed.

Last April, the group made headlines worldwide when it abducted some 220 schoolgirls from Chibok, Borno State. Shortly after, reports emerged that girls were being used in suicide bomb attacks.

In February, at least 10 people were killed by two female suicide bombers at a bus station in Kano state. The same day, another twelve people died when a suicide bomber detonated explosives in a bus in Potiskum, Yobe state.

After witnessing a surge in female suicide bombers, an angry crowd beat to death a young girl suspected of being a potential suicide bomber in Bauchi state.

Recent terror attacks prompted current Nigerian president Goodluck Jonathan to postpone presidential election by six weeks, on grounds of security.

Meanwhile, Nigerian troops were joined by African Union forces in a bid to tackle the terrorists.

Niger and Chard launched a major offensive against the group on 9 March, shortly after Boko Haram's leader Abubakar Shekau pledged allegiance to terror group Islamic State (Isis) in a video.