In a chilling message, the leader of the Islamist militant group Boko Haram has claimed responsibility for the abduction of nearly 300 schoolgirls in the north of Nigeria last month, and said that he would sell them into slavery.

“I abducted your girls,” Boko Haram leader Abubakar Shekau said in the one-hour video released on Monday. “By Allah, I will sell them in the marketplace,” he continued, adding that the girls were being held as “slaves”.

In one of the most shocking attacks in Boko Haram’s five-year insurgency, armed militants swooped on a school in the town of Chibok in Nigeria’s remote northern Borno State on 14 April and snatched 276 girls from their dormitories.

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But it is only now that the group has formally acknowledged responsibility for the brazen attack, a source of deep embarrassment for President Goodluck Jonathan, whose government has been accused of failing to mount a rescue effort for the girls despite receiving regular information of their whereabouts in the days after the attack.

Nigerians all over the country have been united in their anger at the abductions and protesters calling on the government to take action to find the stolen girls have marched in different cities.

In an apparent bid to deflect criticism of the government’s performance, Nigeria’s First Lady allegedly ordered the arrest of Naomi Mutah Nyadar, one of the organisers of last week’s protests in the capital, Abuja.

According to a participant at the all-night meeting on Sunday with Patience Jonathan, the President’s wife accused the protesters of fabricating reports of the kidnappings, and suggested the protest leaders were themselves members of Boko Haram, whose name loosely translates as “Western education is forbidden”.

Saratu Angus Ndirpaya, who attended the meeting, said Mrs 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 Associated Press.

She said that other female allies of Mrs Jonathan at the meeting, including officials of the government and the ruling party, cheered and chanted “yes, yes” when Mrs Jonathan accused them of belonging to Boko Haram.

“They said we are Boko Haram, and that Mrs Nyadar is a member of Boko Haram.” She said Mrs Nyadar and herself do not have daughters among those abducted, but are supporting the mothers of kidnapped daughters.

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The First Lady later denied reports that she had ordered the arrest of Mrs Nyadar, who has since been released, but called on protesters to go home, feeding criticism that the government is not serious about finding the girls.

“You are playing games. Don’t use schoolchildren and women for demonstrations again. Keep it to Borno, let it end there,” she was quoted by the state-owned News Agency of Nigeria as saying.

In response to growing domestic and international pressure, Mr Jonathan promised at the weekend, in his first public comments on the abductions, to mount a search-and-rescue operation for the students, saying that “anywhere the girls are, we will surely get them out”.

He admitted, however, that he did not know where the girls were being held although local media has reported that troops are massing at the edges of the Sambisi forest reserve, Boko Haram’s stronghold.

It is feared, though, that any military operation may already be too late for some of the girls amid reports from local community elders last week that some have already been sold into sex slavery or as “brides” for just £8 in neighbouring countries Chad and Cameroon.

There have also been reports of mass weddings to Boko Haram members, with Christian captives forced to convert to Islam.

Dressed in army fatigues and flanked by an armoured personnel carrier and pick-ups mounted with sub-machine guns, Boko Haram leader Shekau justified the attacks in the video by railing against Western education and values, and efforts to reconcile Muslims and Christians in the sharply divided country.

“I abducted a girl at a Western education school and you are disturbed. I said Western education should end. Girls, you should go and get married,” he said. “I will marry off a woman at the age of 12. I will marry off a girl at the age of nine.

“God instructed me to sell them, they are his properties and I will carry out his instructions,” he added.

The extremist leader went on to threaten similar attacks against schools. Amid fears of attacks by Islamist militants, many schools have been closed for nearly two months in parts of north-east Nigeria, affecting some 120,000 students in a region where secondary school enrolment already stands at just below 5 per cent.

The school in Chibok had been opened only briefly to allow girls from all over the region to take their exams.

Since the schoolgirls were taken last month, authorities say that 53 girls have escaped through their own efforts, either by jumping off trucks or by running away.

By a rough count, about 230 remain in the hands of the insurgents, although locals have warned the actual figure could be higher as there is no definitive list of the girls who were taken.

The kidnappers have said that two of the girls have died from snakebites, while another 20 are reportedly ill.

Since its insurgency began, Boko Haram has carried out increasingly deadly attacks on schools, pupils and teachers in a bid to create a strict Islamic state in the mainly Muslim north of Nigeria.

Some 1,500 people are said to have died in Boko Haram attacks this year alone. The government is seen as doing little to blunt its effectiveness despite an almost doubling of its security budget to £3.6bn in five years.

Most of the group’s attacks have focused on northern Nigeria, but last month the emboldened group set off two bombs in the capital within a fortnight of each other, killing nearly 100 people and highlighting the major security issues that will plague the regional meeting of the World Economic Forum in Abuja this week.

The President has ordered the closure of all schools and government offices during the three-day summit scheduled to open on Wednesday.