Anglo-Dutch oil giant Shell has shut down two key supply pipelines in NIgeria because of leaks and sabotage and declared a "force majeure" on crude oil exports.

Shell's subsidiary in Nigeria SPDC said in a statement the force majeure became effective from Thursday "following the shutdown of both the Trans Niger Pipeline (TNP) and Nembe Creek Trunkline (NCTL)."

The two pipelines take crude to the Bonny Light exports terminal, one of Nigeria's main oil terminals.

The company said a leak was reported on the TNP at Oloma in southern Rivers state, "while the NCTL is shut down for the removal of crude theft points."

Shell said it was working to repair and reopen the two key  pipelines as quickly as possible.

"Force majeure" is a legal term releasing a company from contractual obligations when faced with circumstances beyond its control.

Shell, a major oil operator in Nigeria, did not disclose the volume of output affected by the incident.

The company has blamed repeated oil thefts and sabotage of key pipelines as the major cause of spills and pollution in the oil-producing region.

Crude oil theft or "bunkering" is a major problem in Nigeria, with estimates that the country loses some $6 billion (4.3 billion euros) in revenue every year because of the practice.

Nigeria is Africa's largest oil producer, accounting for more than two million barrels per day.