An explosion at an illegal oil refining depot in Nigeria's Rivers state killed more than 100 people overnight, an environmental group and a local government official said Saturday.

"The fire outbreak occurred at an illegal bunkering site and it affected over 100 people who were burnt beyond recognition," the State Commissioner for Petroleum Resources Goodluck Opiah said.

Unemployment and poverty in the oil-producing Niger Delta have made illegal crude refining an attractive business but with deadly consequences. Crude oil is tapped from a web of pipelines owned by major oil companies and refined into products in makeshift tanks.

The hazardous process has led to many fatal accidents and has polluted a region already blighted by oil spills in farmland, creeks and lagoons.

The Youths and Environmental Advocacy Centre said several vehicles that were in a queue to buy illegal fuel were burnt in the explosion.

At least 25 people, including some children, were killed in an explosion and fire at another illegal refinery in Rivers state in October.

In February, local authorities said they had started a crackdown to try to put a stop to the refining of stolen crude, but with little apparent success.

Government officials estimate that Nigeria, Africa's biggest oil producer and exporter, loses an average of 200,000 barrels per day of oil – more than 10% of production – to those tapping or vandalizing pipelines.

That has forced oil companies to regularly declare force majeure on oil and gas exports.