The BBC's Tomi Oladipo says it is not clear from the video what the fate of the abducted girls is
Nigerian Islamist militant group Boko Haram has threatened to "sell" the hundreds of schoolgirls it abducted three weeks ago.
Militant leader Abubakar Shekau sent a video obtained by the AFP news agency, in which he said for the first time that his group had taken the girls.
About 230 girls are still believed to be missing, prompting widespread criticism of the Nigerian government.
The Boko Haram insurgency has left thousands dead since 2009.
The girls were taken from their school in Chibok, in the northern state of Borno, on the night of 14 April.
Mothers of the missing girls have been speaking to the BBC
Boko Haram, which means "Western education is forbidden" has staged numerous previous attacks on educational institutions in northern Nigeria.
Protest organiser detainedMeanwhile, a woman who helped organise protests over the abduction has been detained, her fellow community leaders say.
Naomi Mutah took part in a meeting called by First Lady Patience Jonathan and was then taken to a police station, they say.
Mrs Jonathan reportedly felt slighted that the mothers of the abducted girls had sent Ms Mutah to the meeting instead of going themselves.
Analysts say Mrs Jonathan is a politically powerful figure in Nigeria.
Ms Mutah, a representative of the Chibok community where the girls were seized from their school, last week organised a protest outside parliament in the capital, Abuja.
The protesters, and many Nigerians, feel the government has not done enough to find the girls.
They were in their final year of school, most of them aged 16 to 18.
President Goodluck Jonathan: "Wherever these girls are we will surely get them out"
Pogo Bitrus, another Chibok community leader, told the BBC he had been to the Asokoro police station where Ms Mutah is reported to have been taken but could find no written record of her being there.
He described the detention as "unfortunate" and "insensitive", adding that he hoped Mrs Jonathan would soon "realise her mistake".
Mr Bitrus noted that Mrs Jonathan has no constitutional power to order arrests.
The AP news agency quotes another community leader, Saratu Angus Ndirpaya, as saying that Mrs Jonathan accused the activists of fabricating the abductions to give the government a bad name.
She also said the First Lady accused them of supporting Boko Haram.
In a TV broadcast on Sunday, his first comment on the abductions, President Goodluck Jonathan said he did not know where the girls were but everything was being done to find them.