Some of the kidnapped Chibok girls have been forced to join Boko Haram and carry out killings, the BBC has been told.

Witnesses who escaped from the same camps that were holding some of the Chibok girls said they have been brainwashed and are being used to terrorise other captives and even carry out killings themselves.

Seventeen-year-old Miriam (not her real name), who fled one of the camps after six months, said the girls were kept in separate houses to the other captives. Sixty-year-old Anna said she had seen Chibok girls with guns, and, like Miriam, seen them commit murder.

In April 2014 shortly after the girls were taken, World Watch Monitor reviewed detailed research compiled from the testimonies of hundreds of women that attempts to explain why Boko Haram strategically targets Christian women and children. This research went back to 1999 and shows that, while the Chibok incident was the one which hit global headlines, it was far from unique. This research has since been submitted by the New York-based Global Justice Center to the Chief Prosecutor of the International Criminal Court in the Hague as part of the submission to examine whether Boko Haram’s abduction of Christian children may constitute genocide, as the “forcible transfer of children.”