"I abducted your girls," the Islamist group's leader Abubakar Shekau said in the 57-minute video obtained by AFP, referring to the 276 students kidnapped three weeks ago from their boarding school in Chibok, Borno state.
The United States said Monday that it is concerned that many of the schoolgirls have been moved out of the country.
State Department spokeswoman Marie Harf shared Washington's assessment after local officials in northeastern Nigerian told AFP the girls had likely been taken to nearby Chad or Cameroon. "Many of them have likely been moved out of the country to neighbouring countries," Harf said, responding to reporters' questions. Harf said Washington provides Nigeria with "counter-terrorism assistance" in the form of intelligence sharing and was standing by to assist "in any way we think that is appropriate".
Fifty-three of the girls managed to escape from the militants but 223 were still being held, state police said last Friday.
Nigeria's President Goodluck Jonathan has been under mounting pressure to act since gunmen stormed the girls' school on April 14, forcing them from their dormitories on to trucks and driving them into the bush. In his first public comments on the abduction on Sunday evening, Jonathan pledged that the government would find the girls and return them to their families. "This is a trying time for this country. It is painful," he said, adding that he had sought international assistance in tackling the country's security challenges.
Hours before the girls were taken, the capital Abuja was hit by its deadliest ever attacks, a car bomb on the outskirts of the city that killed 75 people, also claimed by Boko Haram. A copy-cat bombing in the same spot on May 1 has put the country further on edge as Abuja prepares to host a World Economic Forum summit that opens on Wednesday and includes a visit from Chinese Premier Li Keqiang.
'I will sell them'
In the latest video, Shekau is seen dressed in combat fatigues standing in front of an armoured personnel carrier and two pick-up trucks mounted with sub-machine guns. Six armed men stand beside him with their faces covered.
"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," said Shekau, who speaks in the local Hausa language and Arabic, as well as English. "I will repeat this: Western education should fold up. I abducted your girls. I will sell them in the market, by Allah," Shekau said, adding that his group was holding the girls as "slaves".
"I will marry off a woman at the age of 12. I will marry off a girl at the age of nine," he says at another point in the video.
Unconfirmed reports from local leaders in Chibok suggested the girls had been taken across Nigeria's borders to Chad and Cameroon, and sold as brides for as little as $12.
International attention on the plight of the missing girls was initially slow but has grown quickly in recent days, in part because of a social media campaign – #bringbackourgirls. One of the campaign's leaders, Naomi Mutah, was arrested on Monday during an overnight meeting with First Lady Patience Jonathan for falsely claiming she was a mother of one of the abducted girls. Another of the group's leaders, Hadiza Bala Usman, said Mutah was representing mothers who could not make it to Abuja for the meeting with Jonathan and that the first lady had ordered her arrest.
Jonathan's office denied that the first lady was involved. The reported arrest was widely criticised on social media on Monday and Mutah has since been released, Usman told AFP.
US security warning
Unprecedented security has been put in place for the World Economic Forum meeting, but the kidnapping and twin attacks in Abuja have added to long-standing doubts over Nigeria's ability to contain the insurgency. On Friday, the United States issued a security warning to its citizens in Nigeria to avoid two Sheraton hotels in the financial capital, Lagos, because of an unspecified threat.
US Secretary of State John Kerry vowed on Saturday that Washington would do "everything possible" to help Nigeria deal with Boko Haram militants and bring the children home.
Scores of Boko Haram gunmen riding in armoured vehicles razed a remote northeast Nigerian town of Gamboru Ngala on Monday and opened fire on residents who tried to flee into neighbouring Cameroon, witnesses said. "They have burnt the market, customs office, police station and almost every shop in the town and killed many people but I can't say how many," Musa Abba said.
Boko Haram has been waging an increasingly deadly insurgency in Nigeria's north that has claimed more than 1,500 lives this year alone. The violence has largely been confined to three northeastern states – Borno, Yobe and Adamawa – which have been under a state of emergency since May last year.