DAKAR, Senegal — In a video message apparently made by the leader of the Nigerian Islamist group Boko Haram, Abubakar Shekau claimed responsibility for the kidnapping of hundreds of schoolgirls nearly three weeks ago, called them slaves and threatened to “sell them in the market, by Allah.”

“Western education should end,” Mr. Shekau said in the 57-minute video, speaking in Hausa and Arabic. “Girls, you should go and get married.” The Islamist leader also warned that he would “give their hands in marriage because they are our slaves. We would marry them out at the age of 9. We would marry them out at the age of 12.”

The message was received by news agencies in Nigeria on Monday and is similar to previous videos purportedly from Boko Haram. It is the first time the group has claimed responsibility for the kidnappings, which have gripped Nigeria, ignited a rare antigovernment protest movement and embarrassed the government of President Goodluck Jonathan, who has so far been unable to rescue any of the teenage girls. They were abducted from their school in a remote corner of northeastern Nigeria on April 14. By some counts 276 remain missing.