A leader of the Islamic extremist group Boko Haram has released a video taking responsibility for the kidnapping of 276 Nigerian schoolgirls last month and is threatening to sell the girls on the black market.

"I abducted your girls," Abubakar Shekau said in the video, according to Time. "I will sell them on the market, by Allah."

According to the New York Daily News, on April 15, militants kidnapped more than 300 girls in the middle of the night at a high school in the Chibok area of northeastern Borno, a hotbed for the Boko Haram Islamist group. There are currently 276 still in captivity, while 53 have escaped.

In the video confession, Shekau acknowledged the abduction has caused outrage "because we are holding people [as] slaves."

He also says that the students "will remain slaves with us," which may be a reference to the ancient jihadi custom of enslaving women captured in a holy war and using them as sex slaves.

"They are slaves, and I will sell them because I have the market to sell them," he said.

Protests have erupted around the world demanding the Nigerian government to do more to rescue the victims. On Sunday, President Goodluck Jonathan made a public announcement about the girls in a live TV broadcast. "Wherever these girls are, we'll get them out," said President Jonathan. 

Last week, hundreds of demonstrators marched on parliament in the capital Abuja in "a million-woman march" to raise awareness about the missing girls. The march was also promoted on Twitter under the hashtag #BringBackOurGirls. Many of the marchers carried placards that read "Find Our Daughters."

Parents said the girls are being sold into marriage to Boko Haram militants for $12 each, while human rights groups said they were kidnapped to perform chores and sexual services.

The Boko Haram group, whose name translates to "Western education is a sin" in the local language, opposes the education of women. Their version of Sharia law teaches that women should stay home raising children and waiting on their husbands.