LAGOS, Nigeria -- Nigeria's Islamic extremist leader is threatening to sell the nearly 300 teenage schoolgirls abducted from a school in the remote northeast three weeks ago, in a new videotape received Monday.

Abubakar Shekau for the first time also claimed responsibility for the April 15 mass abduction, warning that his group plans to attack more schools and abduct more girls.

"I abducted your girls," said the leader of Boko Haram, which means "Western education is sinful."

He described the girls as "slaves" and said, "By Allah, I will sell them in the marketplace." The hourlong video starts with fighters lifting automatic rifles and shooting in the air as they chant "Allahu akbar!" or "God is great."

It was unclear if the video was made before or after reports emerged last week that some of the girls have been forced to marry their abductors -- who paid a nominal bride price of $12 -- and that others have been carried into neighbouring Cameroon and Chad. Those reports could not be verified.

In the video, Shekau also said the students "will remain slaves with us." That appears a reference to the ancient jihadi custom of enslaving women captured in a holy war, who then can be used for sex.

"They are slaves and I will sell them because I have the market to sell them," he said, speaking in the Hausa language of northern Nigeria.

The video was reviewed by The Associated Press, and both the face and the voice of the leader of Boko Haram were recognizable.

Shekau brushed off warnings that the abductions could be an international crime, saying in English, as if to reach his accusers in the international community: "What do you know about human rights? You're just claiming human rights (abuses), but you don't know what it is."

An intermediary who has said Boko Haram is ready to negotiate ransoms for the girls also said two of the girls have died of snakebite and about 20 are ill. He said Christians among the girls have been forced to convert to Islam. The man, an Islamic scholar, spoke on condition of anonymity because his position is sensitive.

Nigeria's police have said more than 300 girls were abducted. Of that number, 276 remain in captivity and 53 escaped.

The mass abduction and the military's failure to rescue the girls and young women have ignited national outrage with demonstrations in major cities. Protesters accused President Goodluck Jonathan of insensitivity to the girls' plight.

An organizer of a demonstration said Monday that Jonathan's wife, Patience, ordered the arrests of two protest leaders, accused them of belonging to Boko Haram and expressed doubts there was any kidnapping.

It was unclear what authority Patience Jonathan would have to give such orders, since there is no office of first lady in the Nigerian constitution.

Ayo Adewuyi, spokesman for the first lady, said he was unaware of any arrests. "The first lady did not order the arrest of anybody, and I'm sure of that," he told the AP.

But Saratu Angus Ndirpaya of Chibok town said State Security Service agents drove her and protest leader Naomi Mutah Nyadar to a police station Monday after an all-night meeting at the presidential villa in Abuja, the capital. She said police immediately released her but that Nyadar remains in detention.

By Monday afternoon, police said Nyadar had been returned home. A statement denied she was detained, saying she was "invited ... (to) an interactive and fact-finding interview. "

Ndirpaya said Patience Jonathan accused them of fabricating the abductions. "She told so many lies, that we just wanted the government of Nigeria to have a bad name, that we did not want to support her husband's rule," she said in a telephone interview with the AP.

She said other women at the meeting, allies of Patience Jonathan including officials of the government and the ruling party, cheered and chanted "yes, yes," when the first lady accused them of belonging to Boko Haram. "They said we are Boko Haram, and that Mrs. Nyadar is a member of Boko Haram."

In a report on the meeting, Daily Trust newspaper quoted Patience Jonathan as ordering all Nigerian women to stop protesting, and warning that "should anything happen to them during protests, they should blame themselves."

On Sunday night, President Jonathan said his administration was doing everything possible. On Friday he created a presidential committee to go to the affected Borno state to work with the community on a strategy to free the girls.

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Associated Press writers Lekan Oyekanmi and Bashir Adigun in Abuja, Nigeria, Divine Ntaryike in Douala, Cameroon and Ibrahim Garba in Kano, Nigeria contributed to this report.