The watchdog said it had analysed a video purporting to show soldiers in military fatigues shooting dead the civilians -- footage that a government spokesman dismissed as "fake news".

"The evidence we have provided forms a firm basis for strongly suggesting that the individuals committing these atrocities are members of Cameroon's armed forces," said Samira Daoud, deputy director of Amnesty's West Africa office.

"Some individuals are clearly identifiable and cannot be allowed to get away with such a heinous act with impunity."

The video, posted on social media on Tuesday, shows men wearing uniforms similar to those used by some units in the Cameroonian army forcing two women and their two children to their knees and shooting them.

Before the killings, men speaking in French identified the victims as "BH" for Boko Haram and said they had been captured during an assault against the jihadists, who have mounted operations in several of Nigeria's neighbours since 2015, including suicide bombings.

It has not been possible independently to authenticate or date the video.

But Amnesty said it had studied the footage and matched the military fatigues worn by figures in the video to units in the Cameroon military.

In addition, it said the weapons used in the killing -- Galil rifles -- are comparatively rare in West Africa and are known to be used by the Cameroonian army.

After his government initially dismissed the video as a fake, President Paul Biya on Wednesday ordered an inquiry into the alleged killings.

"It is imperative that a proper, impartial investigation is undertaken and those responsible for these abhorrent acts are brought to justice," said Daoud.

Several NGOs have periodically accused the army in Cameroon of serious atrocities against civilians suspected of having ties with Boko Haram fighters. The military have always denied the allegations.