Armed group claims responsibility for kidnapping 276 girls in Nigeria and threatens to "sell them in the marketplace".

The Nigerian armed group Boko Haram has claimed responsibility for the abduction of 276 schoolgirls during a raid in the village of Chibok in northeast Nigeria last month, the AFP news agency reported, citing a video it had obtained.

"I abducted your girls," the group's leader Abubakar Shekau said on Monday in the 57-minute video obtained by the agency, referring to the hundreds of students kidnapped from their school in Chibok, Borno state, on April 14.

"By Allah, I will sell them in the marketplace," he said in the video that starts with fighters lofting automatic rifles and shooting in the air as they chant "Allahu akbar!" or "God is great".

Boko Haram allegedly stormed the all-girl secondary school, then packed the teenagers, who had been taking exams, onto trucks and disappeared into a remote area along the border with Cameroon.

Boko Haram, now seen as the main security threat to Africa's leading energy producer, is growing bolder and extending its reach.

The apparent lack of capability of the military to prevent the Chibok attack or rescue the abducted girls after three weeks has triggered anger and protests in the northeast and in the capital Abuja.