ABUJA: Nigeria's military was warned of an attack on a school in which more than 200 girls were abducted by Boko Haram militants but failed to act for nearly five hours, Amnesty International said Friday.
The allegation, which the military has denied, came as US, British and French experts arrived on the ground to help trace the schoolgirls and Nigeria said a round-the-clock search was under way.
At least 10 army search teams were trying to track down the girls in the remote far northeast, border guards were on high alert and the air force had so far flown at least 250 sorties.
Nigeria is keen to demonstrate that it is finally acting to trace the 223 girls still missing, after three weeks where the teenagers' parents and families accused them of inaction and indifference.
But Amnesty's claims are likely to heap further pressure on Nigeria's embattled government and military.
Hundreds of people from the girls' home town of Chibok, in northeastern Borno state, took to the streets of the state capital, Maiduguri, to vent their frustrations at the lack of immediate action.
At the same time, Nelson Mandela's widow Graca Machel broke her customary mourning period to plead for the girls' safe return.
Amnesty said that from 7:00 pm local time on April 14, military commanders had repeated warnings about an impending raid in Chibok.
Two senior military officers said not enough troops could be found to head to the town to stave off the attack, as soldiers were reluctant to face guerilla fighters who were better equipped.
Up to 200 armed Boko Haram fighters eventually abducted 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 that Amnesty's allegation was "unfounded, as usual".
"The report is just a collation of rumours," he said.
The girls' kidnap and threat by Boko Haram's leader Abubakar Shekau in a video that he would sell them as slaves has triggered world outrage and a groundswell of calls for action on social networks.
The US team comprises seven military officials from the US Africa regional command AFRICOM, a State Department expert and three FBI personnel, who arrived on Friday, State Department spokeswoman Jen Psaki said.
Four others from the State Department and the USAID aid agency were due to arrive Saturday.
"They'll be providing technical and investigatory assistance, helping with hostage negotiations, advising on military planning and operations and assisting with intelligence and information," she said.
Psaki called the search "challenging" while Secretary of State John Kerry tweeted that it was "too early to conclude" that the girls would be found.
Britain has deployed defence ministry personnel, the Foreign Office said, while French diplomatic sources said a small team was also in Abuja and surveillance equipment was being sent.
China and Interpol have also pledged expert support for the rescue efforts amid growing international awareness of Nigeria's Islamist uprising, which has killed thousands since 2009.
Nigeria is Africa's biggest economy, leading oil producer and most populous nation, with the continent's biggest defence budget by far.
It has in the past resisted security cooperation with the West.
But outrage over the plight of the hostages has prompted President Goodluck Jonathan's administration to welcome offers of assistance, which has been seen as a tacit admission that it requires help to put down the insurgency.
The United Nations refugee agency (UNHCR) said that aside from the kidnappings, focus needed to remain on Boko Haram's wider insurgency.
"The brutality and frequency of (the group's) attacks is unprecedented," it said.
Most of the recent violence has been concentrated in the northeast, where Boko Haram was founded more than a decade ago and more than 1,600 people have already been killed this year.
Attacks in Borno state have occurred with brutal regularity this year. Defenceless civilians are the most frequent victims, forcing them from their homes to seek refuge in other states or neighbouring countries.
Boko Haram has said it is fighting to create a strict religious state in Nigeria's mainly Muslim north.
Some in the deeply conservative northeast have voiced support for such a society.
But experts say 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 killed hundreds in the northeastern town of Gamboru Ngala, on the Cameroon border on Monday.
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