In a video statement obtained by news agency AFP, the leader of the Boko Haram militants Abubakar Shekau said the girls, who were kidnapped from their boarding school three weeks ago, would be sold.

"I abducted a girl at a Western education school and you are disturbed. I said Western education should end. Western education should end. Girls, you should go and get married," Shekau said in the 57-minute video, adding that the group was holding the girls as slaves. "I will sell them in the market, by Allah," he added.

More than 200 girls, aged between 15 and 18, had been kidnapped and, while 53 escaped, more than 276 are still being held captive. The girls had been preparing to sit exams when they were taken from their dormitories in the town of Chibok, in Borno state.

There have been unconfirmed reports that some of them had been forced to marry their captors or were taken to neighboring Chad and Cameroon and sold as brides for $12 (9 euros).

Boko Haram, whose name translates to "Western education is sinful", has been fighting an insurgency in northern Nigeria for the past five years. This year alone, more than 1,500 people have been killed in the violence.

Protest leaders detained

The wife of President Goodluck Jonathan, first lady Patience Jonathan, has been accused by activists on Monday of ordering the detention of protest leaders who were calling for more action from authorities to rescue the teenagers.

Naomi Mutah Nyader, head of the 'Bring Back Our Girls' campaign, led demonstrations over the weekend. Fellow activist Saratu Angue Ndirpaya told the AP news agency that she and Nyadar had been driven to a police station Monday following an all-night meeting with Mrs Jonathan at the presidential villa in the capital, Abuja. Ndirpaya reported that she was released but Nyader remained in custody. Human rights activists told news agency DPA a similar story.

It was unclear what authority Mrs. Jonathan would have to order such detentions. The first lady's office has denied she ordered any arrests.

On Sunday night, President Goodluck Jonathan said his government was doing all it could to rescue the girls. "We promise that, anywhere the girls are, we will surely get them out," he told Nigerians in a live broadcast.

The situation has heightened security fears in Nigeria ahead of its hosting of the World Economic Forum for Africa, due to take place later this week.

se/rc (AFP, Reuters, AP, dpa)