The UN has expressed deep concern over Boko Haram’s threat of selling into slavery hundreds of schoolgirls kidnapped by the group's fighters in Nigeria.

The UN human rights office said on Tuesday that such a move is banned under international law and may constitute a crime against humanity.

“We are deeply concerned about the outrageous claims made in a video believed to be by the leader of Boko Haram in Nigeria yesterday, in which he brazenly says he will sell the abducted schoolgirls ‘in the market’ and ‘marry them off’, referring to them as ‘slaves,’” said Rupert Colville, spokesman for UN rights chief Navi Pillay.

The office has further urged Boko Haram to immediately return the girls to their families unharmed.

“We warn the perpetrators that there is an absolute prohibition against slavery and sexual slavery in international law. These can, under certain circumstances, constitute crimes against humanity. The girls must be immediately returned, unharmed, to their families,” Colville noted.

This comes as the relatives of the girls have been criticizing the Nigerian government for not doing enough to rescue their loved ones. Faced with mounting criticism, the Nigerian President Goodluck Jonathan has pledged to help release the schoolgirls.

Boko Haram fighters abducted 276 girls three weeks ago from their school in Chibok, northern Nigeria.

Several of the captives have managed to escape, but over 220 girls are still being held by Boko Haram.

Several regions of Nigeria have been hit by deadly violence in recent months.

According to the UK-based rights group, Amnesty International, at least 1,500 people have been killed so far this year across Nigeria.