Nigeria's military was warned of an attack on a school in which more than 200 girls were abducted by Boko Haram Islamists 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 in the country 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 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 the 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 7pm 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 11.45pm 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.
Defence spokesman Chris Olukolade told AFP that Amnesty's allegation was "unfounded, as usual".
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.
Along with a US team, UK defence ministry personnel, and a small French contingent, China and Interpol have also pledged expert support.
