A Nigerian militant commander in the oil-rich southern Niger Delta has told the BBC his group is declaring "total war" on all foreign oil interests.
The Movement for the Emancipation of the Niger Delta has given oil companies and their employees until midnight on Friday night to leave the region.
It recently blew up two oil pipelines, held four foreign oil workers hostage and sabotaged two major oilfields.
The group wants greater control of the oil wealth produced on their land.
The warning came as militants and the army exchanged fire after a government helicopter gunship attacked barges allegedly used by smugglers to transport stolen crude oil.
Correspondents say the militants provide security for the smugglers.
Nigeria is Africa's leading oil exporter and the fifth-biggest source of US oil imports, but despite its oil wealth, many Nigerians live in abject poverty.
It is the first time the military leader of the Mend movement, Major-General Godswill Tamuno, has spoken publicly of his group's aims.
He refused to be interviewed on tape or for his location to be disclosed.
He told the BBC's Abdullahi Kaura Abubakar that they had launched their campaign, called "dark February", to ensure that all foreign oil interests left.
He said that they had had enough of the exploitation of their resources and wanted to take total control of the area to get their fair share of the wealth.
Our correspondent says the movement brings together a variety of local Ijaw groups that had been operating in the Niger Delta before.
The group enjoys considerable local support and it is difficult to pinpoint exactly who is a member, he says.
Mend's leaders tend to like to be faceless, our reporter says, and they usually send statements to the media via email.
Shell, one of the oil companies operating in the Niger Delta, told our reporter that security measures were being taken to secure their staff and property, but would not give details.
The Niger delta has been the scene of a low-level war in recent months and the government has increased its military presence in the region.
After a government raid on oil barges earlier this week, Mend released a statement saying the helicopter gunship had fired rockets and machine-guns at targets on land and accused the military of targeting civilians.
It warned that its fighters were capable of shooting down military helicopters and accused Shell of helping out the security forces by allowing them use of an airstrip it operates.
The military has denied it used the facility.
According to AFP news agency, Shell has not confirmed or denied that its airstrip was the base for the attack.
The smugglers are believed to exchange oil for weapons from eastern Europe.
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