Multinational oil giant Royal Dutch/Shell has evacuated non-essential staff from two oil production plants in Nigeria where troops are fighting a major offensive against rebel militia, a spokesman said.
The decision was taken as a precaution after the company noted troop movements on Thursday around the Soku and Ekulama flow stations, which gather oil from wells near the oil city Port Harcourt, he added. Oil production has not been affected.
"We noticed the movement of troops to the Soku and Ekulama areas where we have flow stations and for security reasons we had to evacuate non-essential staff," the spokesman said.
"We have not heard of any exchange of fire, but we sense there may be clashes."
Companies are on a heightened state of alert after a commander of the rebel Niger Delta People's Volunteer Force (NDPVF) told Reuters on Thursday that they would attack oil installations unless the military halted a two-week-old operation to flush out what it calls armed bandits from their river hideouts in the remote area of mangrove swamps and creeks.
Nigerian troops fired on an NDPVF camp using helicopter gunships last week, killing several militants.
Companies fear a repeat of last year's uprising in the nearby Delta state by members of the Ijaw tribe, who are in majority in the region, which forced companies to shut 40 percent of Nigeria's production.
So far, the delta militants have not targeted oil facilities, although a security guard at one Shell flow station near Port Harcourt was killed last month in crossfire.
"The troops are not there to protect our facilities. They are going after militants," the Shell spokesman said.
Nigeria is the world's seventh largest oil exporter and the fifth most important supplier to the United States. About half the nation's 2.5 million barrels per day comes from the eastern delta around Port Harcourt.
The NDPVF has been fighting rival militias in Rivers state since last year, a conflict that observers say is linked to broader political rivalries in the state.
The army announced earlier this month that it was taking over security in Rivers state from the police to "cleanse the state of all forms of armed banditry." Hundreds of extra troops have been moved to places that have seen regular violence.
Amnesty International estimated up to 500 people were killed in fighting in the three weeks to mid-September, but the government says the number is much smaller.
NDPVF leader Mujahid Dokubo-Asari says he is fighting for self-determination for the Niger delta, where most people live in abject poverty despite having all the nation's oil. The government describes him as a bandit fighting for control of smuggling routes used by oil thieves.
The NDPVF say they control the southwestern quarter of Rivers state, where Shell and ENI unit Agip operate oil wells and pipelines.
Oil companies could not immediately specify how much oil production passes through this area, but the whole eastern delta region around Port Harcourt pumps over a million barrels a day.
The most important oil installation in the region is the Bonny export terminal, which exports almost a million barrels per day of crude, but this is unlikely to be threatened in the short term as it is heavily protected by federal troops.