US oil giant ExxonMobil's Nigerian subsidiary warned on Wednesday it may not meet production expectations following the shutdown of a pipeline after an oil spill off southern Akwa Ibom state.

Mobil Producing Nigeria declared "force majeure" and said it had "difficulty in meeting projected liftings because of repair work on a section of pipeline affected in a November 9 oil release incident."

Force majeure is a legal term releasing a company from contractual obligations owing to circumstances beyond its control. ExxonMobil did not say when the measure would be lifted.

It said that it had notified appropriate regulatory agencies and clients.

Nigeria is Africa's biggest 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.

Militants claiming to be fighting for a fairer distribution of oil revenue have also regularly blown up pipelines in the past, though such attacks have greatly decreased since a 2009 amnesty deal.