LAGOS, Nigeria -- The Nigerian military was warned last month that an attack by Boko Haram was imminent several hours before the Islamist militants seized more than 200 girls at a school in a northeast region of the country, according to a report released Friday by Amnesty International.
Despite the warning, Nigerian security forces failed to act, Amnesty International said.
Nigeria's military base in the Borno state capital, Maiduguri, became aware of the imminent attack by Boko Haram on the town of Chibok at just after 7 p.m. on April 14, the human rights group said in a statement, citing "credible sources" it interviewed. By midnight, the attack was under way.
Nigerian security forces had four hours of notice about the attack early on April 15 but did not react because of their fear of engaging the extremists, said Amnesty International in its report "This abduction could have been prevented," Amnesty spokeswoman Susanna Flood said of the Nigerian military's inaction.
The critical report reinforced earlier reporting by AP in which a Chibok official said he had alerted the military of the impending attack but no reinforcements arrived, allowing the insurgents to abduct the schoolgirls.
In wake of the embarrassing revelations, Nigerian military spokesman Maj. Gen. Chris Olukolade didn't answer three calls to his mobile phone Friday seeking comment.
The international effort to rescue the 276 schoolgirls being held captive by Islamic extremists in northeastern Nigeria was boosted Friday when British security experts joined the Nigerian and American forces trying to rescue the missing students.
On Friday, Britain said its aim was not only to help with the current crisis but to defeat Boko Haram.
"The team will be considering not just the recent incidents but also longer-term counter-terrorism solutions to prevent such attacks in the future and defeat Boko Haram," the Foreign & Commonwealth Office said in a statement Friday.
The American team was joined by six additional military officers and more are expected soon, said Pentagon spokesman Navy Rear Adm. John Kirby. The U.S. officers will do a "gap analysis," an assessment to identify what the Nigerian military needs that the U.S. could provide in the search for the girls, he said.
Hostage negotiations is another area where the American team will assist, said U.S. State Department spokeswoman Jen Psaki.
China, France and Spain have also promised help.
Demonstrations in support of the missing Nigerian girls have been held around the world and a social media campaign -- dubbed #BringBackOurGirls -- continued to grow.
In New York, the United Nations Security Council hinted at sanctions against Boko Haram. In a strongly worded statement, the council condemned a May 5 attack that killed and injured hundreds and demanded the immediate release of the kidnapped girls.
Bloomberg News contributed.