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 meters away. "Please don't fire," the man says in pidgin English.
He tries to stand up and get onto the back of a pick up 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.
Video Click Link: http://sg.news.yahoo.com/video/video-shows-nigerian-troops-shooting-220029739.html
"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.
U.S. Assistant Secretary of State for Democracy, Human Rights and Labour Michael Posner said on Friday that the United States was seriously concerned by reported abuses committed by Nigerian security forces in their efforts to quell the insurgency.