Nigeria's military had advance warning of the attack on a school at which some 270 girls were kidnapped but failed to act, a rights group says.

Amnesty International said it was told by several credible sources that the military was given more than fours hours warning of the raid.

Mansur Liman, editor of the BBC's Hausa Service, said the area has had little military presence, despite being under a state of emergency for almost a year.