LAGOS (AFP) - A pipeline in south-western Nigeria has caught fire and exploded in an incident believed to be caused by thieves seeking to steal petrol, but the number of casualties was unclear, officials said on Saturday.

The fire began late Friday on a pipeline operated by state oil firm NNPC which transports petrol in Ogun state, said Ibrahim Farinloye of the National Emergency Management Agency.

Authorities have since shut down the pipeline, but rescuers have not yet been able to access the site to determine if there were casualties, he said.

"NEMA and NNPC wish to confirm that there was an explosion from the pipeline in the area," Farinloye said.

"When the report reached us, we immediately shut down the flow to suffocate supplies to the fire to enable us to put out the fire easily." The incident occurred in the Arepo area of Ogun state.

A number of incidents have occurred in the area involving petrol thieves, including reports of three NNPC staff being killed by suspected vandals last year when they went to the scene following another fire.

Pipeline fires occur regularly in Nigeria, where thieves sabotage them to steal fuel or crude oil for sale on the black market. They often result in high casualty tolls due to the number of people seeking to steal the fuel.

Nigeria is Africa's largest oil producer, but most of its population lives on less than US$2 per day.