Members of Boko Haram have allegedly kidnapped eight more girls aged 12 to 15 years from the northeastern Nigerian village of Warabe, hours after the armed group claimed responsibility for abducting nearly 300 schoolgirls in the town of Chibok, police and residents have said.

A police source, who could not be named, said on Tuesday that the eight girls were taken away overnight on trucks, along with looted livestock and food.

"They were many, and all of them carried guns. They came in two vehicles painted in army colour. They started shooting in our village," said Lazarus Musa, a resident of Warabe.

In a video released on Monday, the armed group threatened to sell the 276 girls abducted on April 14 from a secondary school in Chibok "in the marketplace".

Boko Haram's leader Abubaker Shekau criticised the female students for being taught "western education", which the group is avidly against.

He also warned that his group planned to attack more schools and abduct more girls.

UN warning

He said the girls, some as young as nine-years-old, would be sold for marriage, stating that "God has commanded me to sell".

The statement prompted a warning from the United Nations against "slavery" or "sexual slavery".

"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," UN human rights spokesman Rupert Colville told a news briefing in Geneva.

"That means anyone responsible can be arrested, charged, prosecuted, and jailed at any time in the future. So just because they think they are safe now, they won't necessarily be in two years, five years or 10 years time," he said.

He also urged Nigeria's federal and local authorities to work together to rescue the girls.

On Sunday night, Nigerian President Goodluck Jonathan said his administration was doing everything possible to ensure the schoolgirls were released.

Jonathan welcomed a US offer to send an American team to Nigeria to support the government's response to the kidnapping of more than 200 schoolgirls by an Islamist militant group, the US State Department said on Tuesday.

State Department spokeswoman Jen Psaki said the American embassy in Nigeria is "prepared to form a coordination cell" that would include US military personnel and law enforcement officials with expertise in investigations and hostage negotiations.