A band of hunters who took part in an operation against Boko Haram earlier this month. Photo: AFP

Maiduguri, Nigeria: A new video from Nigeria's home-grown Boko Haram extremists shows gunmen shooting civilians lying face down in a dorm, and a leader saying they are being killed because they are "infidels" or non-believers.

"We have made sure the floor of this hall is turned red with blood, and this is how it is going to be in all future attacks and arrests of infidels," the group leader says.

"From now, killing, slaughtering, destructions and bombing will be our religious duty anywhere we invade."

Most of the bodies appear to be adult men.

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The video comes two days after fleeing villagers reported that the extremists are rounding up elderly people and killing them in two schools in Gwoza, in north-east Nigeria.

The setting of the latest video appears to be a school, a long dormitory furnished with bunk beds which the leader says is in Bama, a town 60 kilometres north of Gwoza.

Students and schools are frequently targeted by Boko Haram, which means "Western education is sinful" in the Hausa language.

Previously, the militants had told residents of villages and towns that they would kill only enemies and wanted people to live peacefully in the area they have dubbed an Islamic caliphate, a large swath along Nigeria's north-eastern border with Cameroon that they have controlled for more than three months.

In the video, the leader notes that the prophet Mohammed advised prisoners should be held, not killed, but says "we felt this is not the right time for us to keep prisoners; that is why we will continue to see that the grounds are crimsoned with the flowing blood of prisoners."

He says some of those killed may call themselves Muslims, but are considered infidels by Boko Haram, a Sunni Jihadi group that imposes strict sharia.

AP