Boko Haram on April 14 stormed an all-girl secondary school in the village of Chibok, in Borno state, then packed the teenagers, who had been taking exams, onto trucks and disappeared into a remote area along the border with Cameroon.
The brazenness and sheer brutality of the school attack shocked Nigerians, who have been growing accustomed to hearing about atrocities in an increasingly bloody five-year-old Islamist insurgency in the north.
I abducted your girls. I will sell them in the market, by Allah, Boko Haram leader Abubakar Shekau said in the video, according to AFP, which is normally the first media outlet to get hold of Shekaus videos.
It did not immediately give further details.
Boko Haram, now seen as the main security threat to Africas leading energy producer, is growing bolder and extending its reach.
The girls abductions have been hugely embarrassing for the government and threaten to completely overshadow its first hosting of the World Economic Forum (WEF) for Africa on May 7-9.
The apparent powerlessness of the military or inability to find the girls in three weeks has triggered anger and protests in the northeast and in Abuja.
On Sunday, authorities arrested a leader of a protest staged last week in Abuja that had called on them to do more to find the girls. The arrest has further fuelled outrage against the security forces.
Naomi Mutah Nyadar was picked up by police after a meeting she and other campaigners had held with President Goodluck Jonathans wife, Patience, concerning the girls.
Police were not immediately available to comment, but a presidency source said Nyadar had been detained because she had falsely claimed to be the mother of one of the missing girls. Abana said she had made no such claim.