The deaths highlight the worsening power crisis in Africa's top oil producer, where the near collapse of the national grid is forcing homes and businesses to turn to portable generators. Much of Nigeria has no mains power for weeks at a time. The capacity of the world's eighth biggest oil exporter has plunged to less than 1,000 megawatts from 3,000 a year ago.
The victims fell asleep in a locked room with the generator still running, police said. Their bodies were discovered and the incident reported.
"We are still investigating the cause of the deaths. But a power generating set was found in the hall where they slept, so we are not ruling out suffocation through carbon monoxide inhalation," police spokesman Ali Okechukwu said.
The family had gathered in a village in the Isiala-Ngwa district of Abia to pray for one of their own whom they said was being haunted by evil spirits.
Though Nigerians are very religious, many are highly superstitious and hold strong beliefs in voodoo, ghosts and witchcraft. One survivor, Linus Abba, said the victims were attacked by the evil spirits they were trying to exorcise.
"I heard strange noises and noticed that the room where we slept was filled with smoke, that is all I can remember because I fainted," Abba told Reuters. "It was a terrible attack by the forces of darkness that we came to fight."
Generator fumes kill scores of people in Africa's most populous country of 140 million every year.