Gunmen using speedboats have clashed with Nigerian soldiers as they attacked an oil platform operated by Shell in the Niger Delta.
Shell staff have started to leave the area
A Shell spokesman said five workers were injured in the attack on the Benisede oil station in Bayelsa state.
An unknown number of attackers, as well as soldiers defending the platform, were killed, a military official said.
In response, Shell has started withdrawing personnel from the facility and from neighbouring flow stations.
An unnamed industry source told Reuters news agency the Anglo-Dutch company was considering a complete staff withdrawal from the west of the delta region.
Correspondents say the move will increase pressure on Nigeria's government to crack down on ethnic Ijaw militants who want more control over the region's oil revenues.
Elsewhere in the Delta last week, a major pipeline that feeds an export terminal was ruptured by militants.
The kidnappings and explosion, the latest in a string of violent incidents in the troubled region, have slashed Shell 's production there by some 220,000 barrels a day - almost 10% of Nigeria's average output of 2.6m barrels.
In a statement about the latest attack, Shell said "heavily armed persons" attacked the platform early on Sunday.
"The attackers invaded the flow station in speedboats, burnt down two staff accommodation blocks, damaged the processing facilities and left," it added.
The injured members of staff were taken to hospital.
Soldiers guarding the platform returned fire, said Brig Gen Elias Zamani, commander of a special task force assigned to the area.
A number of soldiers and assailants died in the attack, he said.
On Wednesday gunmen kidnapped four foreign workers from another Shell oil rig in the Niger delta region.
The hostages, who are still being held, come from the UK, the US, Honduras and Bulgaria, a Shell spokeswoman said.
The Nigerian army, which has thousands of troops in the Delta, has been unable to prevent attacks by militants who object to the country's oil wealth leaving their region.
Nigeria is Africa's leading oil exporter and the fifth-biggest source of US oil imports.