Anglo-Dutch oil group Shell said Monday it has shut in 25,000 barrels per day of crude in a southern Nigerian oil field due to spills caused by sabotage and theft.
"The Shell Petroleum Development Company of Nigeria Ltd (SPDC) has shut in production from Imo River Field due to a recent upsurge of illegal bunkering and refining activities which have impacted the environment," the company said in a statement.
"Some 25,000 barrels of oil per day is affected," Shell said, adding it took the action on August 28 "to starve the illegal bunkerers of crude oil in order to prevent further environmental pollution."
Theft of crude is commonly referred to as "bunkering" in Nigeria.
Shell said the field which straddles oil-producing Abia and Rivers states, has five flow stations, a gas compressor station and several kilometres of pipelines among other faculties.
The thefts were first noticed in the field two years ago, prompting government security forces to move in to dislodge the perpetrators and destroyed their barges and canoes.
The military deployed in the oil delta has in recent months reported busting hundreds of illegal refineries.
"But the criminal act has now resumed, with crude thieves inflicting hacksaw cuts on pipelines to siphon crude to waiting barges and canoes, some of which can hold as much as 40,000 barrels," it added.
Pipeline vandalism and associated spills are common in the Niger Delta, Nigeria's oil and gas producing region, to feed a lucrative black market.
Shell, one of Nigeria's major oil operators, has seen frequent shut-ins in recent years.
Last month, it was forced to shut down its Utorogu gas plant following a leak on its trunkline in western Niger Delta.
Activists say oil firms such as Shell have not done enough to prevent such incidents.
A UN report in August said decades of oil pollution in the Ogoniland area of the Niger Delta, located in Rivers state, may require the world's largest ever cleanup.
Nigeria is Africa's largest oil producer, accounting for 2.3 million barrels of crude per day according to the latest report of the International Energy Agency.