The Nigerian Islamist group Boko Haram has released a video purporting to show some of the kidnapped Chibok schoolgirls.

Some 50 girls wearing headscarves are seen behind a Boko Haram militant who demands the release of fighters in return for the release of the girls.

The group seized 276 girls from their school in the northern town of Chibok, of which 219 are believed to remain in captivity.

This is the third video said to show the girls since they were captured.