Boko Haram militants pretending to be soldiers slaughtered at least 200 civilians in three villages in northeastern Nigeria, witnesses said on Thursday.
A community leader who witnessed the killings and wished to remain anonymous said the military failed to intervene even though it was warned that an attack was imminent.
He said residents of the Gwoza local government district in Borno state had pleaded for the military to send soldiers to protect the area after being warned of the impending raid, but help was not sent.
Instead, a group of the Islamist militants arrived in Toyota Hilux pickup trucks, commonly used by the military – and told the group they were soldiers "and we are here to protect you all".
They then gathered residents into the centre, before shouting "Allahu Akbar, Allahu Akbar" and firing at the crowd.
The community leader said he survived the massacre because he “was going round to inform people that the soldiers had come and they wanted to address us".
In pictures: Nigeria kidnapped schoolgirls
In pictures: Nigeria kidnapped schoolgirls
A total of 276 girls were abducted from the northeastern town of Chibok, in Borno state, which has a sizeable Christian community. Some 223 are still missing
One of the kidnapped girls looks into a camera
One of the missing girls talking to the camera
The missing Nigerian schoolgirls, wearing the full-length hijab and praying in an undisclosed rural location. Boko Haram alleging they had converted them to Islam
Girls wearing the full-length hijab holding a flag reading "There is no god, but Allah" and "Mohammed is Allah's prophet"
A man claiming to be the leader of Nigerian Islamist extremist group Boko Haram Abubakar Shekau
Abubakar Shekau speaks on the video
Girls, wearing the full-length hijab and praying are filmed by an unidentified man (R) in an undisclosed rural location
People carry signs as they attend a protest demanding the release of abducted secondary school girls in the remote village of Chibok in Lagos
A protester demonstrates against the kidnapping of school girls in Nigeria, outside the Nigerian Embassy in London
Chief International Correspondent Christiane Amanpour and Prime Minister David Cameron appearing on the BBC1 current affairs programme
People participate in a "Bring Back Our Girls" campaign demonstration and candlelight vigil in Los Angeles
Girls holding heart shaped banners in a "Bring Back Our Girls" campaign demonstration and candlelight vigil in Los Angeles
14/19 South Africa
South Africans protest in solidarity against the abduction of hundreds of schoolgirls in Nigeria by the Muslim extremist group Boko Haram and what protesters said was the failure of the Nigerian government and international community to rescue them, during a march to the Nigerian Consulate in Johannesburg
Karilyn Coates (10) joins others in a candlelight vigil for the more than 300 girls abducted by Boko Haram in Nigeria, at All Souls Unitarian Church in Colorado Springs
Mothers of the missing Chibok school girls abducted by Boko Haram Islamists gather to receive informations from officials. Nigeria's president said that Boko Haram's mass abduction of more than 200 schoolgirls would mark a turning point in the battle against the Islamists, as world powers joined the search to rescue the hostages
Former Nigerian Education Minister and Vice-President of the World Bank's Africa division (3rd L) Obiageli Ezekwesilieze speaks as she leads a march of Nigeria women and mothers of the kidnapped girls of Chibok, calling for their freedom in Abuja
18/19 Bring Back Our Girls
Kelly Hoppen tweeted: 'Please make sure you do this, we must stand together and not forget them'
19/19 Bring Back Our Girls
E.L. Rock Star tweeted: 'Join The Movement'
The slaughter was confirmed by both Mohammed Ali Ndume, a senator representing Borno and whose hometown is Gwoza, and by a top security official in Maiduguri.
It took a few days for survivors to get word of the massacres to Maiduguri, the provincial capital, because travel on the roads is extremely dangerous and phone connections are poor or nonexistent, the community leader said. The death toll from Monday's attacks is among the highest to date.
The Nigerian government is currently under pressure to secure the release of more than 200 school girls who were kidnapped by Boko Haram in April. Since the kidnapping, Boko Haram has increased its attacks on small towns and cities.
The mass kidnapping and bombings have piled political pressure on President Goodluck Jonathan, who on Thursday ordered a "full-scale operation" against Boko Haram.
Of the 276 girls the militants kidnapped, 57 escaped in the early days of the abduction, according to officials in the Borno state government.
A USAID-backed inter-faith group arrived in Maiduguri on Wednesday to offer psychiatric help to the girls' parents.
Additional reporting by Associated Press and Reuters