About 230 girls are still believed to be missing, prompting widespread criticism of the Nigerian government.
“I abducted your girls,” Boko Haram leader Abubakar Shekau said in a video obtained by the AFP news agency.
Meanwhile, a woman who helped organise protests over the abduction has been detained, her fellow community leaders say.
Naomi Mutah took part in a meeting on Sunday called by the president’s wife, Patience Jonathan. Shortly afterwards, she was taken to a police station.