Armed with assault rifles, fighters in speed boats invaded the Idama platform operated by Chevron in the southern Niger Delta, while in the capital a judge ordered Mujahid Dokubo-Asari to be held in jail for two weeks pending treason charges.

"Eight boats, each carrying 15 armed people, occupied the Idama flow station. Six government security forces had their weapons taken from them," a source close to Chevron said.

"Apparently the militants are now heading for more stations. The situation can only get worse."

Only 8,400 barrels per day were shut down at Idama, Chevron said, but industry officials said the impact could rise dramatically if the attacks spread.

Commanders of Asari's Niger Delta People's Volunteer Force (NDPVF) threatened to blow up oil facilities across the delta, which accounts for almost all of Nigeria's 2.4 million barrels per day production, unless their leader was released.

"We are going to blow up all flow stations and pipelines from Warri to Calabar if Asari is not back in 48 hours," said Dakuro Princewill, a NDPVF commander, in reference to the eastern and western extremities of the vast wetlands region.

Militants loyal to Asari burned tyres in the streets of the delta's largest city, Port Harcourt, and blocked a major road artery. Police shot in the air to disperse them.

Royal Dutch Shell withdrew about 50 non-essential staff from three oil and gas facilities, a senior industry source said, but its 950,000 barrels per day of oil output was unaffected.

Italian oil company Agip, a unit of ENI, also withdrew some staff from Port Harcourt, the source said. Agip spokespeople were not available for comment.

Oil prices are at near record highs due to hurricane damage in southern United States, and any disruption to exports of high quality crude from Nigeria, its fifth-largest supplier, would stretch supplies further.

Threats

Asari campaigns for self-determination of his Ijaw tribe, the largest in the delta, and argues that the colonial treaties that created the union with the rest of Nigeria are fraudulent.

The government has called him an oil thief and gangster.

An Abuja high court on Thursday granted a request by the justice minister to detain him for two weeks to prepare charges of treason, which carries a maximum death penalty, and unlawful assembly.

"The provocation did not come from Asari but from the Nigerian state ... If anything happens they should not hold us responsible. He has supporters everywhere. We cannot control them," said Onegiya Erekosima, a spokesman for NDPVF.

Police brought Asari to the court house briefly after the detention order was given. He said he had been held incommunicado and his lawyer had been arrested.

"If this is what (President Olusegun) Obasanjo claims is democracy, it is the highest dictatorship," Asari said.

Despite its oil resources, most inhabitants of the delta live in poverty and feel cheated out of their wealth.

The resentment fuels armed conflicts, sabotage of oil installations, kidnappings of oil workers and oil theft.

Last year, Asari and hundreds of his followers fought gun battles with troops from rebel hideouts in the creeks near Port Harcourt, until a peace deal was signed which gave Asari amnesty in return for disarmament.

NDPVF commander Princewill said he had about 3,000 volunteers ready to fight and they would be able to procure arms when they needed them.