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 boarding school in Chibok, in the northern state of Borno, on the night of 14 April.
Boko Haram, which means "Western education is forbidden", has attacked numerous educational institutions in northern Nigeria.
'God instructed me'
In the video, Abubakar Shekau said the girls should not have been in school in the first place, but rather should get married.
"God instructed me to sell them, they are his properties and I will carry out his instructions," he said.
However, BBC Hausa Service editor Mansur Liman points out that the Boko Haram leader did not state the number of girls abducted, nor where they were taken or are now.
Assurances from President Goodluck Jonathan have done little to convince Nigerians of the government's commitment to freeing the girls, says our correspondent.
The Associated Press news agency says it is unclear whether the video was made before or after reports last week that some of the girls had been forced to marry their abductors, who paid a nominal bride price of $12 (£7).
Others are reported to have been taken across borders into Cameroon and Chad.
The girls were in their final year of school, most of them aged 16 to 18.
The BBC Hausa Service has received reports of a gun battle on the Nigeria-Cameroon border, and houses being burnt down by individuals suspected to be members of Boko Haram.
No further details are available.
Protest organiser detained
Meanwhile, a woman who helped organise protests over the abduction was detained and later released.
Naomi Mutah was taken to a police station after a meeting called by First Lady Patience Jonathan.
Mrs Jonathan reportedly felt slighted that the girls' mothers had sent Ms Mutah to the meeting instead of going themselves.
Mrs Jonathan is seen as a politically powerful figure in Nigeria but has no constitutional power to order arrests.
Ms Mutah, a representative of the Chibok community, organised a protest last week outside parliament in Abuja.
Pogo Bitrus, another Chibok leader, described Ms Mutah's detention as "insensitive", telling the BBC he hoped Mrs Jonathan would soon "realise her mistake".
The AP news agency quotes another community leader, Saratu Angus Ndirpaya, as saying that Mrs Jonathan accused the activists of fabricating the abductions and supporting Boko Haram.
In a TV broadcast on Sunday, his first comment on the abductions, President Jonathan said he did not know where the girls were but everything was being done to find them.