Protests have been held across Nigeria, demanding more action to rescue the girls. Photo: BBC

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.

A presidency source quoted by the Reuters news agency said Ms Mutah had been detained for allegedly and falsely claiming to be the mother of one of the missing girls.

But one of her fellow protesters told Reuters Ms Mutah was calling on the authorities to "bring back our daughters" in the broader sense of "daughters of Nigeria".

"They are so clueless," Lawan Abana said.

The girls were abducted on 14 March from their school in Chibok, in the northern state of Borno.