LONDON -- Amnesty International alleged Friday that Nigerian security officials failed to act on an advance warning about a militant group's raid on a boarding school that led to the abduction of more than 300 girls.

The human rights organization cited "multiple interviews with credible sources," in asserting that Nigerian security forces had four hours of notice about the April 15 attack by Boko Haram in northeastern Nigeria. Amnesty says an inability to muster troops and fear of engaging with better equipped forces prevented forces from being deployed.

"This abduction could have been prevented," Amnesty spokeswoman Susanna Flood said.

More than 300 girls were initially abducted, with 53 of them escaping later. At least 276 are still missing.

The Nigerian military's failure to find the girls has drawn international attention to an escalating Islamic extremist insurrection that has killed more than 1,500 people so far this year. Boko Haram, which wants to impose Islamic law on Nigeria, has threatened to sell the girls.

President Goodluck Jonathan's government stands accused of being slow to mount operations to rescue the girls. British experts travelled to Nigeria on Friday to help as an international effort began taking shape.

Boko Haram, whose name means "Western education is sinful," abducted the girls from a boarding school in the northeast town of Chibok. Amnesty said that local civilian patrols in the neighbouring village of Gagilam raised the alarm when men on motorbikes entered their community.

Two senior officers in Nigeria's armed forces told Amnesty they were aware of the planned attack even prior to the calls received from local officials. One officer, whose name was not given, was quoted by Amnesty as saying that soldiers are afraid to go to the battlefronts.

"The fact that Nigerian security forces knew about Boko Haram's impending raid, but failed to take the immediate action needed to stop it, will only amplify the national and international outcry at this horrific crime," said Netsanet Belay, Amnesty's Africa director.