Kidnapped Schoolgirls 'In At Least Four Groups'

Intelligence sources have told Sky News they believe they know where some of the schoolgirls kidnapped in Nigeria are.

The sources believe they have been split into at least four different groups, complicating the search to find the girls almost four weeks after they were taken by militants from Boko Haram.

British and American officials are using advanced eavesdropping equipment to scan the Sambisa forest where the schoolgirls are thought to be, Sky News understands.

Sky's Special Correspondent Alex Crawford, who is in the Nigerian capital, Abuja, said if the intelligence is true it would be "much more difficult to mount simultaneous raids" to rescue them.

"The whole thing is fraught with danger," she added.

It has been claimed Nigerian security forces failed to respond to warnings about Boko Haram's planned abduction of the 276 girls from a boarding school in Chibok in the northern Borno state on April 14.

Amnesty International said it had verified reports from several credible sources who claimed the military was aware of the impending attack close to four hours before it took place.

The Nigerian government has rejected the findings as "unfounded".

The leader of Boko Haram, Abubakar Shekau, has threatened to sell the girls "on the market", amid reports some have already been trafficked to neighbouring Chad and Cameroon.

But Nigerian President Goodluck Jonathan said he believes the girls are still in the country.

A British team of experts sent to "advise and support" the rescue operation touched down in Abuja earlier on Friday.

The Foreign and Commonwealth Office says Britain's aim is not only to find the girls but to eradicate Boko Haram.

Prime Minister David Cameron has told Sky News the abduction is "a ghastly situation , an act of pure evil".

The Islamist militant group's five-year insurgency has left at least 1,500 dead this year alone.

Its name is said to figuratively mean "Western education is forbidden".

US first lady Michelle Obama is to speak out against the kidnappings later today when she gives the weekly presidential address instead of her husband, the White House has said.

Earlier this week, she expressed solidarity with the kidnap victims, tweeting a photograph of herself holding a placard bearing the slogan #BringBackOurGirls.