Abubakar Shekau claimed in a video obtained by the news wire service that a buyer for the schoolgirls would be found.

He said: "I abducted your girls. I will sell them in the market, by Allah."

At least 223 youngsters were seized by suspected militants during a raid in the village of Chibok, in Borno state, north Nigeria, three weeks ago. 

Reports had previously circulated that some of the girls missing have already been sold as brides across Nigeria's border with Chad and Cameroon for as little as €8.50 ($12).

Police and the families say the number still being held is as high as 276.

The brazenness of the school attack - during which the girls were rounded up and put on to trucks - has shocked Nigerians accustomed to atrocities in a five-year conflict with insurgents.

The girls were abducted from Borno state three weeks ago

President Goodluck Jonathan took to the radio and TV on Sunday night to respond to claims his government was not doing enough to recover the girls.

He said: "We promise that anywhere the girls are, we will surely get them out."

The president said he had asked US President Barack Obama, Britain, France and China, for help to counter the activities of the Islamic terror group Boko Haram.

And he dismissed claims his government was negotiating with Boko Haram.

He also pleaded for the parents of the missing girls and their local communities to co-operate with the rescue efforts during what he described as a "trying" and "painful" time.

It came amid claims that Nigeria's First Lady - Goodluck Jonathan's wife Patience - had ordered the arrest of two leaders of a protest demanding more action from the government.

First Lady and President of Nigeria Patience and Goodluck Jonathan

One of the protest leaders said Patience Jonathan abused them, expressed doubts there was any kidnapping and accused them of belonging to Boko Haram.

Saratu Angus Ndirpaya said Mrs Jonathan accused them of giving Nigeria's government and her husband "a bad name", at a meeting at the presidential villa in Abuja. 

Ms Ndirpaya said State Security Service agents then drove her and another protest leader Naomi Mutah Nyadar to a police station, before later releasing Ms Ndirpaya.

AP journalists waiting outside the police station in Abuja where Ms Nyadar was apparently being held, saw the protest leader bundled into a presidential car and driven away.

Ayo Adewuyi, spokesman for first lady Patience Jonathan, told AP there was a meeting but he was unaware of any arrests.

"The first lady did not order the arrest of anybody, and I'm sure of that," he said.