In a press statement, government spokesman Issa Tchiroma Bakary late Wednesday condemned the mobile phone video as a "horrible forgery" and "fake news" targeting the army in its campaign against Boko Haram extemists from Nigeria.

"Nevertheless (...), the head of state has ordered the opening of an investigation in accordance with relevant practices," the minister added.

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

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.

Two Cameroonian security sources said it may have been filmed in the mountains of the Far North Region, across the border from some Boko Haram strongholds in Nigeria.

Tchiroma said that the "countryside and the relief clearly indicate that this is in the Sahel-Saharan zone. In such circumstances, the combat uniform used by the Cameroonian army is always of the Sahel type.

"The men concerned (in the video) wear mottled battledress, forest gear in some cases."

The minister added that voice-overs in the video had been "loosely edited on to visual sequences" and dismissed the movie as "an unfortunate attempt at twisting reality and brainwashing the public".

A non-governmental organisation, the Network of Defenders of Human Rights in Central Africa, known by its French acronym Rhedac, claims to have cross-checked details in the video and to attest to "its authenticity".

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.