Shell have suggested sabotage is responsible for the Ogoniland spill
Oil production at an oil installation off the Nigerian coast remains halted because staff are suffering from shock after a militant attack, officials say.
The attack on the Royal Dutch Shell facility stopped about 10% of Nigeria's oil production. Shell also said the militants had damaged equipment.
An American hostage was released after a few hours.
It is the first attack on an offshore facility, previously thought safe despite a wave of inshore attacks.
Shell also said the emergency shutdown might have damaged equipment, which would have to be repaired.
The raid took place on the Bonga oil platform about 120km (75 miles) off the coast of the Niger Delta.
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 and previously cut Nigeria's output by about 20%.
The Movement for the Emancipation of the Niger Delta (Mend) claimed it carried out the attack in an e-mail sent to journalists.
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.
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.
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 sq ft of gas per day
Oil and gas drawn up from 16 well heads on the ocean floor to a processing tanker
Now they have proven that in terms of distance at least, all of Nigeria's facilities are within their reach.
Local activists in the Ogoni region have asked Shell to come and contain the oil spill that has covered farmland.
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.