The video shows policemen lined up on a hillside with their hands tied behind their backs in front of gunmen who berate them and then open fire on them, the BBC reports.
A note released alongside the video said that the policemen were captured in Pakistan's volatile Dir district when militants crossed the border from Afghanistan in June.
Pakistan's army spokesman said the video appears to be authentic.
The bodies were discovered by locals once the militants had left.
The BBC received mobile phone footage from a policeman in Dir showing local residents discovering the corpses.
The area is regularly targeted by insurgents and hundreds of militants were reported to have descended on the town of Shalato in a remote corner of Dir close to the border with Afghanistan's Kunar province.