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| The school in Chibok. |
Nigeria's military had advance warning of the attack on a school at which some 270 girls were kidnapped but failed to act, a rights group says.
Amnesty International says it was told by several credible sources that the military was given more than fours hours' warning of the raid.
Fifty-three of the girls escaped shortly after being seized on 14 April, leaving more than 200 still captive.
The authorities have not yet commented on Amnesty's statement.
Teams of experts from the US and UK have arrived in Nigeria to help locate and rescue the abductees.
It says one reason was a "reported fear of engaging with the often better-equipped armed groups".
It is believed the schoolgirls are being held somewhere in the vast forested areas that stretch from near Chibok into neighbouring Cameroon.
Amnesty says a small contingent of security forces in Chibok was overpowered by the attackers.
Militant Islamist group Boko Haram has said it captured the girls, saying they should not have been in school and should get married instead.
Boko Haram, whose name means "Western education is forbidden" in the Hausa language, began its insurgency in Borno state in 2009.
At least 1,200 people are estimated to have died in the violence this year alone.
In a video released earlier this week, Boko Haram leader Abubakar Shekau threatened to "sell" the students.
Fifty-three of the girls escaped shortly after being seized on 14 April, leaving more than 200 still captive.
The authorities have not yet commented on Amnesty's statement.
Teams of experts from the US and UK have arrived in Nigeria to help locate and rescue the abductees.
Amnesty says the military in the Borno State capital, Maiduguri, was informed of the impending
attack soon after 1900 local time - the school was attacked around midnight.
Despite the warning, reinforcements were not sent to help protect the school in the remote Chibok area, Amnesty says.attack soon after 1900 local time - the school was attacked around midnight.
It says one reason was a "reported fear of engaging with the often better-equipped armed groups".
It is believed the schoolgirls are being held somewhere in the vast forested areas that stretch from near Chibok into neighbouring Cameroon.
Amnesty says a small contingent of security forces in Chibok was overpowered by the attackers.
Militant Islamist group Boko Haram has said it captured the girls, saying they should not have been in school and should get married instead.
Boko Haram, whose name means "Western education is forbidden" in the Hausa language, began its insurgency in Borno state in 2009.
At least 1,200 people are estimated to have died in the violence this year alone.
In a video released earlier this week, Boko Haram leader Abubakar Shekau threatened to "sell" the students.
SourceBBC