The Nigerian border town with Chad, Gamboru Ngala, was left unguarded after the Chadian troops liberated it from and flushed out the insurgents, reports say.
Speaking under the condition of anonymity, the source, who claimed that it was a reprisal, said many Nigerians from the area had fled to Kusihri village in Cameroon.
He said the insurgents stormed Gamboru Ngala town, setting many homes ablaze while shooting indiscriminately, leading to the death of 20 persons.
Quoting members of the youth vigilance groups in the area, the source added that the youths heard heavy gunshots from all angles, with many of them fleeing to Cameroon.
One of them, he said, told him on the telephone that he saw 15 corpses lying on the path with many residents of the area whereabouts unknown.
