Royal Dutch Shell said it shut down production at an offshore oil installation that produces about 200,000 barrels per day after the most powerful militant group in Nigeria said it launched an attack there Thursday.
A leader of the Movement for the Emancipation of the Niger Delta told The Associated Press that militants attacked the Bonga oil field more than 65 miles from land.
But the fighters weren't able to enter a computer control room, which they had hoped to destroy.
The militant leader spoke on condition of anonymity to avoid punishment by authorities.
Olav Ljosne, a spokesman for Royal Dutch Shell, confirmed an attack, but gave no details.
He said production had been stopped from the field.
The militants also said they kidnapped a foreign worker from a supply vessel they met while returning home from the attack, but there was no immediate confirmation of that.
Attacks against offshore facilities are exceedingly rare.
Oil industry officials consider their operations on the high seas much safer than those in the creeks and swamps of Nigeria's southern Niger Delta, where most of the attacks during two years of increased violence have taken place.
Militant attacks on oil infrastructure have trimmed about a quarter of total oil production in Nigeria, which is Africa's biggest producer and a member of OPEC.
The turmoil in Nigeria's south has helped send oil prices to historical heights, giving the militants more leverage in their drive to force the federal government to send more oil industry proceeds to their areas.
Despite being the home of almost all of Nigeria's petroleum reserves, the country's south is as desperately poor as the rest of the country, which is Africa's most populous with 140 million people.
But criminality and militancy are closely linked, with many of the militant groups accused of stealing crude oil from wells and pipelines for sale in overseas market and helping politicians rig elections.