An explosion at an illegal oil refinery has killed at least 110 people, police and government officials said on Sunday.

The blast occurred late on Friday at the facility, which lies between the southern Nigerian oil states of Rivers and Imo, police confirmed.

What do we know about the casualties?

Ifeanyi Nnaji of the National Emergency Management Agency (NEMA) told AFP news agency that the death toll had risen from 80 as dozens of people "succumbed to their injuries."

Ifeanyi who heads NEMA operations in the area, said scores of people "with severe burns are still in the hospitals."

A mass burial is being planned for those killed in the explosion, many of who "were burnt beyond recognition," said Declan Emelumba, the Imo State Commissioner for Information.

What caused the blast?

Although the reason for the blast is unknown, it might have occurred during the process of refining crude oil, which involves boiling it by means of fire.

Idris Musa, director general of Nigeria's National Oil Spill Detection and Response Agency said the dead included those engaged in illegal oil refining and bunkering.

Musa said an investigation into the disaster was underway.

Nigerian President Muhammadu Buhari described the incident as a "catastrophe and a national disaster."

The sponsors of the illegal refinery "must all be caught and made to face justice," he added through a statement from his office.

The Nigerian leader also ordered the security and intelligence agencies to intensify the clampdown on illegal refineries in the country.

What happens at illegal oil refineries?

Oil thieves, vandals and illegal refining operators siphon off some 200,000 barrels of crude oil daily in Nigeria, according to industry operators.

The oil is tapped from a web of pipelines owned by major oil companies and refined into products in makeshift tanks at the illegal refineries, which flourish among poverty that is widespread despite the country being the biggest oil producer on the continent.

The refineries, scattered among the creeks, swamps and waters of the oil-rich Niger Delta, are not only prone to accidents but also cause spills and pollution.

Pipeline fires are also commonplace in Nigeria, partly because of poor maintenance but also because of acts of vandalism by people who siphon off petrol and sell it on the black market.

A pipeline blast in the southern town of Jesse in October 1998 killed more than 1,000 villagers, with the government blaming the disaster on such thieves, though local residents said the pipeline was not maintained properly.

Nigeria has a crude oil output of around two million barrels per day.

mm, tj/aw (AFP, dpa)