Nigeria's military has warned an armed group in its oil-producing area it will take off the "kid gloves" unless the militia stops threatening oil workers.
Clashes between rebels and security forces have escalated
The militia said foreign oil companies must cease production or face "all-out war" in the Niger Delta from 1 October.
Anglo-Dutch oil company, Shell, the biggest oil company in Nigeria, has boosted security following the threats.
Nigeria is the world's seventh largest exporter of oil, but 70% of the population live in poverty.
Human rights group Amnesty International has said that the army already had orders to use "maximum force" in the area, leading to the deaths of up to 500 people in the past six weeks.
The militia says it is fighting for the liberation of the Ijaw people. Local authorities say they are merely oil thieves and dismiss their threat.
Fears of the Nigeria unrest spreading were one reason why oil prices have reached a record high of more than $50 a barrel, traders say.
A statement from Shell described the region as still tense and said that the movement of employees and supplies had been curtailed.
Oil production has also been affected.
The Santa Barbara flow station, which produces 28,000 barrels per day, has been shut down because the company cannot reach the area to fix a technical problem.
Dokubo Asari, the leader of the Niger Delta People's Volunteer Force, told the BBC that all foreign nationals should withdraw from the region with immediate effect.
He said his group would not take responsibility for any harm that befalls a foreigner after his release of a communique threatening to escalate violence.
He added that expatriates - who in this region are predominantly oil workers - could only return when fundamental issues of resource control and self-determination had been resolved.
Dokubo Asari took to the creeks of River State earlier this year, and hundreds of people have died in the subsequent clashes with the police, navy and rival gangs.
Fighting has intensified in the last month after the military launched a major operation against the group.
On Thursday Shell pulled out more than 200 of its non-essential staff from two gas and oil fields because of heightened tensions.
A spokesman for Rivers State government called Dokubo Asari a "joker".
"He does not have the capacity to destroy oil installations. The government will not allow it. The security forces are fully mobilised and combat-ready to dislodge this criminal group," Emmanuel Okah told AFP news agency.
The BBC's Anna Borzello in Nigeria says the Niger Delta holds the bulk of Nigeria's oil reserves, but the area is under-developed and riven by conflict, often caused by armed gangs involved in the lucrative trade of smuggling crude oil.