The kidnapping of more than 200 Nigerian girls is an "outrage" according to the United States, which has promised help to try to find them.

The pledge came after the children's Islamist kidnapper reportedly vowed to sell them.

In a video obtained by the AFP news agency, Boko Haram leader Abubakar Shekau described the students as "slaves," adding: "I abducted your girls. I will sell them in the market, by Allah."

At least 276 youngsters were seized from their school in the village of Chibok, in Borno state, north Nigeria, by militants on April 14.

Nigerian police said 53 managed to escape shortly after the attack, but at least 223 are still missing. Relatives suggest that figure could be considerably higher.

White House spokesman Jay Carney said: "We view what has happened in Nigeria as an outrage and a terrible tragedy.

"The President has been briefed, several times, and his national security team continues to monitor the situation there closely.

"The State Department has been in regular touch with the Nigerian government about what we might do to support its efforts to find and free these young women."

Reports previously said some of the missing girls have already been sold as brides across Nigeria's border with Chad and Cameroon for as little as £7 ($12).

It is not clear whether the video was recorded before or after those reports.

The video was the first time Boko Haram - regarded by the US as a terrorist organisation - has admitted carrying out the abduction.

The group, whose name means 'Western education is sinful', regularly attacks civilian targets.

However, the brazenness of the school attack has shocked Nigerians accustomed to atrocities in the five-year conflict with the insurgents.

A group called Bring Back Our Girls, comprising relatives of the missing students, have staged regular protests across Nigeria calling for the government and military to do more to find the girls.

Nigerian President Goodluck Jonathan said: "We promise that anywhere the girls are, we'll surely get them out."

He said he had asked the US, Britain, France and China for help to counter Boko Haram and dismissed claims his government was negotiating with the group.

It came amid claims that protest leader Naomi Mutah Nyadar was arrested after holding a meeting with Nigeria's First Lady, Patience Jonathan, at the presidential palace.

Fellow protester Hadiza Bala Usman said Ms Nyadar was arrested "at the request of the first lady" after falsely identifying herself as one of the mothers of the girls.

Another, Lawan Abana, said Ms Nyadar was in fact representing mothers who could not make it to the meeting in Nigeria's capital Abuja and had never claimed otherwise.

Protester Saratu Angus Ndirpaya added that Patience Jonathan had abused the women gathered at the presidential palace and expressed doubts there was any kidnapping. She also allegedly accused them of belonging to Boko Haram.

The first lady's office has issued a statement denying the allegations.

Ms Nyadar has since reportedly been released.