Nigeria's army has committed atrocities in the north-east in its fight against Islamist militants belonging to Boko Haram, Amnesty International says.
The campaign group said it had seen "gruesome footage" including alleged members of the military slitting the throats of detainees.
The Nigerian authorities said such barbarity had no place in the military and promised to investigate.
Boko Haram has been waging an insurgency in Nigeria since 2009.
Thousands of people have been killed in a series of bombings and assassinations in the north-east and in the capital, Abuja, this year alone.
Amnesty says footage obtained from multiple sources on a trip to Borno state, in the north, "includes horrific images of detainees having their throats slit one by one and dumped in mass graves".
The perpetrators "appear to be members of the Nigerian military and the "Civilian Joint Task Force" (CJTF), state-sponsored militias," the organisation adds.
"The ghastly images are backed up by the numerous.testimonies we have gathered which suggest that
extrajudicial executions are, in fact, regularly carried out by the Nigerian military and CJTF," says Amnesty
International Secretary General Salil Shetty.
The Nigerian authorities say they are "deeply concerned" about the footage in circulation, adding: "That level of
barbarism and impunity has no place in the Nigerian military."