At least 12 people were killed and 40 others injured in northeast Nigeria when an explosive device detonated, civilian vigilantes helping the military against Boko Haram told AFP on Wednesday.

The device was picked up by one of the vigilantes as they travelled back to the Borno state town of Monguno from a military operation against the Islamists in Marte, 40 kilometres (26 miles) away.

"We lost 12 members in the accidental explosion, which also injured 40 others," vigilante leader Abdullahi Amadu Bukar told AFP from the Borno state capital, Maiduguri.

"We have brought seven of the dead to Maiduguri this morning (Wednesday) and left five others behind in Monguno."

The vigilantes had been fighting the insurgents alongside troops in Marte when one of them picked up the explosive device and brought it back to Monguno.

It exploded late on Tuesday as the civilian fighters were examining it on the outskirts of the town, said Bukar, whose account was backed up by another vigilante, Awwal Ibrahim.

Nigeria's military claimed to have liberated Marte and Monguno in February after a ground and air campaign but Bukar said the insurgents had returned to Marte once troops withdrew.

Elsewhere, a military truck accompanying technicians from a Nigerian power company to make repairs in the Damboa district of Borno on Monday hit a roadside bomb believed to have planted by the rebels.

Three soldiers were killed in the blast, forcing the team to abandon the mission and returned to Maiduguri, 86 kilometres away, according to local vigilante sources.