Oil producer Shell says it has shut down a field off the coast of Nigeria after a leak during a transfer of oil to a tanker led to 40,000 barrels spilling into the Atlantic Ocean.
The Bonga field is approximately 120km (75 miles) offshore and produces 10% of Nigeria's oil exports.
A spokesman for Shell said the flow of oil from the leak had now stopped and no-one had been injured.
The Anglo-Dutch company says it is focusing on the clean-up.
The head of Shell Nigeria, Mutiu Sunmonu, apologised for the leak.
"As soon as we became aware of it, we stopped the flow of oil and mobilised our own resources, as well as industry expertise, to ensure its effects are minimised," he said.
"It is important to stress that this was not a well control incident of any sort, and to make clear that no-one has been injured."
Most previous oil spills in Nigeria have been onshore and caused by sabotage or militant attacks.
In August 2011 a UN report into spills in Ogoniland found that the region could take 30 years to recover.
The BBC's Fidelis Mbah, who is in Port Harcourt, says local environmental campaigners are afraid that the spill may have devastating effects on fishing communities close to the oil field.
Nigeria is one of the world's biggest oil producers, and the government announced plans to increase production earlier this month.