There was no immediate claim of responsibility from the Movement for the Emancipation of the Niger Delta (Mend), which has been carrying out attacks on oil installations since early 2006 as part of a campaign for greater autonomy and local control over oil wealth, or any other group.

However, in an e-mailed statement, Mend said: "No Mend units or affiliated groups have reported such heavy losses."

Armed men have previiously attempted to overrun military patrols with the aim of stealing military equipment.

Delta threat

Meanwhile, The Niger Delta Vigilante, another armed group with links to Mend, threatened more violence in the region.

"We are by this notice asking all oil companies and foreigners to leave Port Harcourt because there is no going back," said the group's spokesman, who uses the pseudonym Tamunokuro Ebitari.

"We are bringing the fight to their door post and we will not spare anybody or companies."

The group said on Monday that its "diving unit" had attacked a pipeline owned by state-oil firm NNPC, that supplies natural gas to the Alakiri flow station.

Officials with Nigeria's Joint Task Force, NNPC and Shell could not confirm the attack.

Nigeria's oil facilities frequently come under attack from armed groups.

About one fifth of the country's oil production has been shut down in the last two years. Until recently Nigeria was Africa's leading oil exporter but the security situation has now pushed it into second place behind Angola.