The Nigerian Islamist group Boko Haram has released a video showing some of the schoolgirls it abducted two years ago from the northern town of Chibok.

Some 50 girls are shown with a gunman who demands the release of fighters in return for the girls. He says some girls died in air strikes.

In this report, from the BBC's Martin Patience in Nigeria, the images of the girls are obscured, to protect their identities.