The blast happened in the southern River state's Emohua district, where illegal refineries are common. Residents said the death toll was likely to grow because many of the bodies were burned beyond recognition and dozens of people were injured.
Police confirmed the incident but did not provide details of what happened. Locals said most of the people who died had worked at the ilegal refinery in the village of Rumucholu.
The workers at the site were refining oil taken from a vandalized pipe, according to local sources.
The explosion is the latest in a series of deadly accidents at illegal oil refineries in Nigeria. In August, an explosion at an illegal refinery in the state of Imo killed at least 20 people.
Illegal oil refining is a major problem in Nigeria's Niger Delta region, where poverty and unemployment are widespread. The refineries are often poorly constructed and operate in unsafe conditions, making them prone to explosions and fires.
The Nigerian government has made efforts to crack down on illegal oil refining, but the problem persists. (ILKHA)