The footage shows the victims with their hands tied behind their backs as the gunmen in front of them open fire at them, the BBC reported. A Pakistani army spokesman told the BBC the video appeared to be authentic.
The BBC said a note with the video claimed the policemen were captured during a cross-border raid from Afghanistan in June.
The four gunmen firing at the policemen are shown with their faces covered, The New York Times reported. Prior to the execution, one insurgent speaking in Pashtu language accuses the victims of killing six children in Pakistan's Swat district as per the translation posted by the Long War Journal, the report said.
"These are the enemies of Islam who originated from Pakistan," the speaker is heard saying.
"They are the Pakistani police, soldiers and their supporters who recently lined up six kids in Swat and shot them execution-style," the insurgent says. "These Pakistanis are now our captives, and we will avenge the death of the children by doing the same to them."
Those words are followed by gunfire and the men fall to the ground. Later, the report said, an insurgent inspects each man and fires at those still alive. Another insurgent is seen taking a video of the executions.
The video was first shown on the LiveLeak video-sharing Web site, the report said.
The BBC said the bodies were found by local people after the militants left the scene.
Though the Pakistani military denied it, human rights advocates have said there is enough evidence the Pakistani army also has been responsible for summary executions of suspected Taliban militants, the BBC reported.
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