* Exxon one of the largest operators in Nigeria
* U.S. oil firm says not sure the cause of leak
* Shell, Eni have outages onshore due to floods
By Joe Brock
ABUJA, Nov 10 (Reuters) - Exxon has shut a pipeline off the coast of Nigeria's Akwa Ibom state after an oil leak started by an unknown cause, the company's local unit said on Saturday.
The U.S. major's outage will add to production problems in Africa's biggest crude exporter, after fellow oil majors Shell and Eni reported recent disruptions at onshore sites due to Nigeria's worst flooding in 50 years.
"(Exxon Nigeria) confirms that on Nov. 9 an oil release occurred offshore Akwa Ibom State," Mobil Producing Nigeria, a joint venture between Exxon and the state oil firm, said in an emailed statement.
"The source of the leak was identified and the pipeline was isolated and shutdown." The company said it was investigating the cause of the leak but didn't give any details on the amount of oil production lost.
There was an oil spill in August near an Exxon facility that residents said left a slick running for miles along the coastline of Akwa Ibom. Exxon said it cleared up the spill but didn't confirm the source of the leak.
Italian oil firm Eni said on Friday it had declared force majeure on Brass River oil loadings from Nigeria due to floods, which have submerged part of the southern oil-producing Niger Delta in recent weeks.
Flooding combined with oil theft, prompted Shell to declare force majeure on two other large Nigerian oil streams, Bonny Light and Forcados, in late October.
Oil spills are common in Nigeria's onshore Niger Delta due to widespread theft by oil gangs tapping into pipelines and the poor maintenance of some ageing infrastructure.
But offshore spills are less common. Last December, an accident at Shell's offshore Bonga facility spilled an estimated 40,000 barrels, one of the largest in Nigeria's history.
Nigerian regulators told parliament in July that Shell should be fined $5 billion for environmental damaged caused by the spill but the company has said there is no legal basis for the fine.