An explosion and fire near an illegal oil refinery site in Nigeria’s Niger Delta region killed at least 12 people Friday, police said, although local residents reported a much higher death toll.

The explosion in Emuoha council area of the southern Rivers state occurred along a pipeline targeted by illegal refinery operators who were trying to steal oil, state police spokesperson Grace Iringe-Koko said.

“We are aware that there was an explosion relating to bunkering activities,” she said, adding that authorities were working to determine the number of casualties and the cause of the incident.

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People in the area told The Associated Press that dozens may have died in the fire that raged for hours and that the victims were mostly young people who planned to siphon oil from a pipeline and to transport to an illegal refinery site in at least five vehicles.

Fyneface Dumnamene, executive director of Youths and Environmental Advocacy Centre, said a spark from the exhaust pipe of a bus loaded with gallons of crude oil ignited the explosion as the driver attempted to depart.

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“Everybody in about five vehicles there was all burnt,” Dumnamene told the AP.

Illegal refineries are a lucrative business in Nigeria, one of Africa’s top oil producers. They are more rampant in the oil-rich Niger Delta region, where most of the nation’s oil facilities are located.

The workers at such facilities rarely adhere to safety standards, leading to frequent fires, including one in Imo state last year in which more than 100 people were killed.

Nigeria lost at least $3 billion worth of crude oil to theft between January 2021 and February 2022. Shady business operators often avoid regulators by setting up refineries in remote areas such as the one in Imo, the Nigerian Upstream Petroleum Regulatory Commission (NUPRC) said last year.