A video obtained by Reuters shows Nigerian troops shooting unarmed captives in broad daylight by the roadside in the northeastern city of Maiduguri, the bastion of an Islamist insurgency.
Nigeria's military has long been accused of human rights abuses, including summary executions, in the troubled north but there has been no video proof since the first crackdown on the Islamist sect Boko Haram in 2009.
A spokesman for the army said it was "impossible" for Nigerian troops to do such a thing.
Boko Haram is fighting to carve an Islamic state out of Nigeria, and its fighters have killed hundreds in bomb and gun attacks, many of them from the security forces, since beginning the uprising three years ago.
The video was taken by a soldier who said he was present while the shootings took place two weeks ago. The soldier, who requested anonymity, passed it to Reuters on Sunday.
In the grainy footage, a man sits down next to three or four corpses piled together on the roadside. He pleads for his life while soldiers shout at him and a crowd looks on a few metres away. "Please don't fire," the man says in pidgin English.
He tries to stand up and get onto the back of a pickup truck to the left. A Nigerian soldier shouts "come out", and drags him off it, shoving him on the ground.
One of them kicks him in the head. Then he and another soldier aim assault rifles at him. Four gunshots are heard and the man lies still next to the others.
Nigerian army spokesman Colonel Mohammed Yerima said he had not seen the video but that the events must have been staged.
"How can they do that? It is not possible. This is the Boko Haram tactics," He said. "They will do the killing, say it's the military and then Amnesty International and so on will blame us. It's not possible for Nigerian troops to act in this way."
Nigerian forces have repeatedly denied accusations of such abuses, saying the only times they kill suspected militants is during combat. Those captured are questioned or freed, they say.
US Assistant Secretary of State for Democracy, Human Rights and Labour Michael Posner said that the United States was seriously concerned by reported abuses committed by Nigerian security forces in their efforts to quell the insurgency.
Such alleged abuses usually occur shortly after members of the security forces have been killed or wounded in an attack by the sect. The killings in this video happened after a bomb attack on a military patrol further up the road, the soldier who provided the footage said.
Another video from the same source, which he said was taken after the executions, shows soldiers piling up about two dozen bodies in two bloody heaps on the ground from the back of a military truck.
The videos could spur renewed calls for Nigeria's security forces to change their approach to the insurgency, which critics say is prompting desperate, angry youths to join Boko Haram and encouraging the northern population to shelter them.
That uprising was sparked by a military crackdown on the sect in which hundreds were killed, including its founder and spiritual leader Mohammed Yusuf, who died in police custody.
President Goodluck Jonathan has been accused of treating the conflict as a security problem that can be solved with force alone, rather than addressing the root causes of the insurgency.
Amnesty International issued a report this month in which it said human rights abuses committed by security forces were fuelling the conflict they were meant to end.
The report said a "significant number" of people accused of links with Boko Haram had been executed after arrest without due process, while hundreds were detained without charge or trial and many of those arrested disappeared or were later found dead.
The Nigerian military rejected that report, including accusations that they execute suspects, as "biased and mischievous."