The UN Security Council strongly condemned the abduction of hundreds of Nigerian schoolgirls, as the world body's secretary general dispatched an envoy to help find them. The diplomatic moves came after 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 round-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. The 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 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 (1800 GMT) 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 guerrilla 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. 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. 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. 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. - Brutal regularity - 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. 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 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.