Militants have led a violent campaign in the Niger Delta
Oil company Royal Dutch Shell says it has temporarily stopped production at its main offshore oilfield in Nigeria, following a militant attack.
The raid took place overnight on the Bonga oil platform about 120km (75 miles) off the coast of the Niger Delta, the company said.
It is the first attack on the oilfield, which normally produces about 200,000 barrels a day.
Shell has also been blamed for an oil spill in the Ogoni region of the Delta.
Oil is gushing from disused pipes abandoned by the company when it left the region nearly 15 years ago, following local protests.
Attacks on the inshore Niger Delta have helped drive up world oil prices.
Nigeria's valuable offshore oilfields had always been considered difficult for most militants to target, the BBC's Alex Last reports from Lagos.
But early on Thursday, gunmen in boats reached the Bonga installation, Shell's flagship project, for the first time.
A Nigerian navy spokesman confirmed reports that militants had kidnapped a US oil worker from a separate vessel on their way back from the raid.
The Movement for the Emancipation of the Niger Delta (Mend) claimed it carried out the attack in an e-mail sent to journalists.
An American hostage taken from another boat during the attack was released after several hours.
The gunmen failed to get inside the installation's control room, but attacked a key vessel used for production storage and offloading, a Shell spokesman said.
Several people were reported to have been injured. Mend says it is campaigning for a greater share of the region's oil wealth to be kept by local people, but the government says they are criminals, motivated by the ransoms they receive from oil companies.
The shutdown has cut a tenth of Nigeria's total output in one go.
This comes on top of a reduction of at least 20% in recent years following inland attacks.
Discovered in 1995
Production began in 2005
Expected to last until 2019
120Km off the Nigerian coast
Capable of producing 200,000 barrels of oil and 150 million cubic ft of gas per day
Oil and gas drawn up from 16 well heads on the ocean floor to a processing tanker
Our correspondent says Bonga was new, expensive and working well despite the difficulties and repeated attacks affecting the company's inshore operations in the Delta.
The militants in the Delta are getting more sophisticated and better equipped and armed, he says.
Now they have proven that in terms of distance at least, all of Nigeria's facilities are within their reach.
Next month, a peace summit organised by the government on the Niger Delta unrest is due to begin in the capital, Abuja.
The government has promised amnesties to any militants who take part.
Mend has refused to attend unless Henry Okah, one of their leaders currently on trial for treason and gun-running, is also granted amnesty.
But the government has refused.
"We want everyone to be there to air their grievances," vice-presidential aide Akilu Indabawa told the BBC's Hausa Service.
"But in Henry Okah's case the issue is in front of a court and the government can't intervene because it respects the rule of law."
Local activists in the Ogoni region have asked Shell to come and contain the oil spill that has covered farmland.
Shell have suggested sabotage is responsible for the Ogoniland spill
The yellow brown oil is flowing through the village of Kpor and into a stream about a mile away.
Villagers told the BBC they heard a "thunderous noise" and ran to the spot to see oil spraying all over their land.
Last week the government revoked Shell's rights to drill for oil in Ogoni saying the company had "lost the trust" of the local community.
Shell stopped drilling there in 1993 after pressure from the Movement for the Survival of the Ogoni People (Mosop) and it has not returned since.
A Shell spokesman said in the past such spills have been because saboteurs damaged sealed well heads.