A video has been released showing Nigerian troops shooting unarmed captives in broad daylight by the roadside in Maiduguri, the bastion of an Islamist insurgency.

Nigeria's military has long been accused of human rights abuses, including summary executions, but there has been no video proof since a crackdown on the Islamist sect Boko Haram in 2009.

A spokesman for the army said it was "impossible" for Nigerian troops to do such a thing.

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Boko Haram wants to carve an Islamic state out of Nigeria and its fighters have killed hundreds, many from the security forces, since starting the uprising three years ago.