Clashes between rebels and security forces have escalated
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Foreign workers in Nigeria's oil-rich Niger Delta have been warned to leave the region by a local militia.
The group said foreign oil companies must cease production or face "all-out war" from 1 October.
The militia says it is fighting for the liberation of the Ijaw people. Local authorities say they merely oil thieves and dismiss their threat.
Nigeria is the world's seventh largest exporter of oil, but 70% of the population live in poverty.
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.
But a spokesman for Shell, the biggest oil company in Nigeria, said the firm was undeterred by the threat.
"We are not in any way moved by the threat. We believe the Nigerian security forces are equal to the task of safeguarding oil installations and protecting workers," said Don Boham.
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Anyone who assists the Nigerian state to make money in
Ijawland will be... targeted
Militia leader Dokubo Asari
In pictures: Fighting for oil
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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.
Fighting intensified
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.
However, Shell says production has not been affected.
A spokesman for Rivers State government also dismissed the threat.
"Dokubo-Asari is 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.