Witness Haruna Bukar says the civilians were patrolling when they came across the bag of metallic objects and carried them to the nearby town of Monguno, where they exploded yesterday.
He says many curious people had gathered to inspect the objects when they detonated. Nigeria's homegrown extremists routinely use improvised explosive devices in their attacks.
Boko Haram had taken control of a large swath of northeast Nigeria until a multinational force this year forced them out of towns and villages. Nigeria's military says the extremists are confined to the Sambisa Forest.