Gunmen attacked an offshore facility operated by US oil giant Exxon Mobil off the coast of southern Nigeria at the weekend, the company said Monday.
The Nigerian subsidiary of Exxon Mobil said one of its offshore facilities "was boarded by unknown armed persons in the evening of Sunday," but gave no further details.
The raid is the latest on oil facilities in Akwa Ibom state, one of Nigeria's main oil producing states.
Seven foreign workers were captured last week in waters off the same state when gunmen attacked a rig and support vessel operated by British-based Afren.
A statement purportedly from Nigeria's main militant group in the oil producing region, the Movement for the Emancipation of the Niger Delta (MEND) claimed responsibility for last week's attack.
In another statement, MEND last week warned of a series of attacks on oil installations across the Niger Delta in the coming days.
However, the statements came from a different email address than the militant group has used previously to warn of and to claim attacks.
MEND has carried out scores of attacks in the oil-producing Niger Delta region in recent years, slashing output and playing havoc with oil prices.
It also claimed responsibility for twin car bombings on independence day on October 1 that killed at least 12 people, the first such attack in the Nigerian capital Abuja.
The group claims to be fighting on behalf of local people in the deeply impoverished Niger Delta, but it has also been seen as an umbrella organisation for criminal gangs in the volatile oil region.
Exxon Mobil said "relevant government and security agencies have been informed and appropriate response measures are underway at this time."
Following a spate of attacks including on politicians' properties and kidnappings in the region, the military at the weekend warned it would clampdown on militant camps.
"We have observed with concern some criminal acts within the past few days, by some people claiming to be militants," chief of defence staff, Air Marshal Oluseyi Petirin, said.
"Many of these criminals are known to be hiding in camps within the creeks of Niger Delta. These camps will no longer be tolerated," said the military chief telling villagers living in the vicinity of the camps to leave immediately "to avoid any collateral damage".
Nearly a dozen major incidents of attacks and kidnappings have occurred in the restive delta situated in south of the OPEC member country and the world's eighth oil exporter.
Thousands of militants last year laid down arms under a government amnesty offer resulting in a lull in attacks lasting several months.
Since July, there has been an upsurge in attacks ahead of crunch general elections due early next year.
Armed men last week used explosives to attack the house of a top presidential adviser on the Niger Delta crisis and overseer of the government's amnesty programme for repentant rebels in the region.