US oil giant ExxonMobil's Nigerian subsidiary on Saturday said a pipeline had been shut down after an oil spill off southern Akwa Ibom state.

The source of Friday's offshore leak had been identified and an investigation launched, the company said without giving further details.

Nigeria is Africa's largest oil producer. Its oil-producing Niger Delta region has been badly contaminated by decades of spills.

Spills occur frequently in the region due to a range of factors, including sabotage by oil thieves or militants as well as operational or maintenance faults by the oil companies.