Nigeria's military was warned of an attack on a school at which more than 200 girls were abducted by Islamists but failed to take action, Amnesty International claimed on Friday.

The claim, which the military has denied, came as foreign experts arrived on the ground in Nigeria to help trace the schoolgirls abducted by Boko Haram.

Amnesty said that from 7:00 pm (1800 GMT) on April 14, the authorities failed to act on repeated warnings about the impending raid in the remote town of Chibok, in northeastern Borno state.

Two senior military officers interviewed by the global rights monitor said the commander was unable to muster enough troops to head to the town to stave off the attack.

Up to 200 armed Boko Haram fighters abducted a total of 276 girls at about 11:45 pm after fighting a small number of police and soldiers stationed in the town.

Amnesty's Africa director for research and advocacy, Netsanet Belay, described the situation as a "gross dereliction of Nigeria's duty to protect civilians", adding that people remained "sitting ducks" for future attacks.

"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," he said.

"The Nigerian leadership must now use all lawful means at their disposal to secure the girls' safe release and ensure nothing like this can happen again."

Defence spokesman Chris Olukolade told AFP Amnesty's "allegation is unfounded as usual".

"The report is just a collation of rumours," he said.

The US embassy in Abuja told AFP that a team of its experts had arrived in Nigeria, without specifying the make-up of the group.

British specialists, including defence ministry personnel, had also landed, the Foreign Office said.

The US and Britain are among a number of countries, as well as France and China which have pledged expert support, with the kidnapping raising international awareness about an Islamist uprising that has killed thousands since 2009.

Nigeria had initially been slow to respond to the kidnappings and the military's search and rescue effort has been fiercely criticised by activists and parents of the hostages.

But a series of protests in the capital, a growing global social media campaign, and attention from world leaders and celebrities has put pressure on Nigeria to act more aggressively.

Outrage in particular has come after Boko Haram leader Abubakar Shekau threatened to sell the girls as slaves.

Amnesty said the teenagers' abduction and detention were war crimes.

Nigeria has in the past resisted security cooperation with the West, experts said, but amid outrage over the plight of the hostages, President Goodluck Jonathan's administration this week welcomed offers of assistance.

The United Nations refugee agency (UNHCR) said that aside from the kidnappings which have captured global attention, focus needed to remain on Boko Haram's wider insurgency.

"The brutality and frequency of (the group's) attacks is unprecedented," UNHCR said in a statement.

Most of the group's recent violence has been concentrated in the remote northeast, where Boko Haram was founded more than a decade ago, and where more than 1,600 people have already been killed this year.

Attacks in Borno state have at times seemed a weekly occurence this year, with defenceless civilians the most frequent victims.

"Some have witnessed friends or family members being randomly singled out and killed in the streets," UNHCR said.

"People speak of homes and fields being burned to the ground, with villages completely razed, or grenades being launched into crowded markets killing people and livestock," the statement added.

Boko Haram has said it is fighting to create a strict Islamic state in Nigeria's mainly Muslim north.

Some in the deeply conservative northeast have voiced support for a society governed by sharia, or Islamic law.

But experts say that any public support Boko Haram may have once had in the region has been largely destroyed by its ruthless campaign against civilians.

The most recent massacre by the group killed hundreds in the northeastern town of Gamboru Ngala, on the Cameroon border.

Islamist gunmen on Monday razed much of the town and fired indiscriminately on civilians as they tried to flee, burning entire families in their homes.

Jonathan said on Thursday that the Chibok kidnappings would mark a turning point in the battle against the Islamists, calling it "the beginning of the end of terrorism in Nigeria".

Nigeria's failure to contain the violence has raised questions as to whether the country can eliminate Boko Haram without outside help.

Nigeria is Africa's most populous country and top economy, with by far the largest defence budget in west Africa.

But its military has been widely accused of committing serious rights abuses and its competence has been criticised.

Some have voiced hope that collaborating on the hostage rescue may improve Nigeria's broader capacity to defeat Boko Haram.

Britain's foreign office said that aside from working to rescue the hostages, its team would also be focused on "longer-term counter-terrorism solutions to prevent such attacks in the future and defeat Boko Haram."