AMNESTY International said that Nigeria’s military had been warned of an attack on a school in which more than 200 girls were abducted by Boko Haram Islamists three weeks ago, but failed to act for nearly five hours.

The Nigerian military denied the allegation and has launched a around-the-clock search for the missing girls.

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 allegations are likely to heap further pressure on Nigeria’s embattled government and military.

A unanimous UN Security Council declaration said the mass kidnappings “may amount to crimes against humanity” under international law, but made no explicit reference to charges in the International Criminal Court.

The 15 members of the council said they would follow the situation and consider “appropriate measures” to take against Boko Haram.

The statement urged their immediate release, without conditions. Meanwhile, UN Secretary General Ban Ki-moon announced he was sending his special representative for West Africa, Said Djinnit, to Abuja to discuss ways of helping authorities locate the girls.

US, British and French experts have already arrived on the ground to help trace the schoolgirls.

DERELICTION OF DUTY

Hundreds of people from the girls’ home town of Chibok, in north-eastern 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 7:00pm 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 11:45pm 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”.

INTERNATIONAL SUPPORT

The girls’ kidnap and the 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.

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

Earlier this week, she expressed solidarity with the kidnap victims, tweeting a photograph of herself holding a placard bearing the slogan #BringBackOurGirls, the hashtag used as the rallying cry for a viral campaign calling for the schoolgirls’ return.

You can read more of the world’s tweets on the issue right here.