It also announced the arrest of seven soldiers in connection with a separate video appearing to show men in military uniform shooting dead two women and two children.
Amnesty said in a statement it verified the latest video through witness testimony, satellite imagery and analysis of the weapons, dialogue and uniforms. It said the video was shot in Achigaya at an unknown date prior to May 2016.
The nearly four-minute video circulated on social media in recent days. A government spokesman said it was released to undermine President Paul Biya ahead of an election in October.
At the start of the video, several men in military fatigues with automatic-style weapons joke among themselves, with one saying in French: “This is a kamikaze mission.” The men then open fire for 12 seconds on about a dozen people sitting or lying down against a wall and under guard.
After the firing stops, one armed man approaches the motionless bodies and fires several more times from point-blank range. Around them, buildings in the village are in flames and gunfire can be heard in the background.
“Here is yet more credible evidence to support allegations that Cameroon’s armed forces committed grave crimes against civilians,” said Ilaria Allegrozzi, Amnesty International Lake Chad Researcher.
Government spokesman Issa Tchiroma Bakary said authorities would open an investigation into the video but government was the victim of a “campaign of denigration” before the election, when Biya will try to extend his 36-year rule.
“People want to discredit the army and president,” Tchiroma told Reuters.
Tchiroma said in a statement authorities arrested seven soldiers, including a lieutenant and a sergeant, in connection with the video. The statement did not say when the arrests took place. Reuters reported on July 19, citing security sources, four soldiers were arrested in connection with the video — information authorities later disputed.
“The Minister of Communication reiterates the Head of State’s resolve to ensure atrocities that may be committed by a few misguided soldiers are systematically investigated and, if need be, appropriate actions meted out,” it said.
Government and army officials previously dismissed the video as “fake news” meant to tarnish government’s image, even as they promised to investigate.
The Cameroonian army’s elite Rapid Intervention Brigade (BIR) has been battling Nigerian militant group Boko Haram in the country’s extreme north since late 2014.
The militants killed hundreds in the Far North and their tactics include the use of child suicide bombers to attack checkpoints, market places and mosques.