Boko Haram has claimed responsibility for the abduction of hundreds of schoolgirls in northern Nigeria that has triggered international outrage, threatening to sell them as “slaves”.

“I abducted your girls,” the Islamist group’s leader Abubakar Shekau said in the 57-minute video, referring to the 276 students kidnapped three weeks ago from their boarding school in Chibok, Borno state.

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, including from the US, in tackling the country’s security challenges.

Hours before the girls were taken, the capital Abuja was hit by its deadliest ever attack: 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.

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.

“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,” he said.

“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 $US12.