The US firm Chevron shut down a station pumping 19,000 barrels a day yesterday after being warned of an "imminent threat" from armed militants.

A day earlier more than 100 fighters stormed another Chevron oil platform, disarming government security personnel and shutting down the facility, which pumped around 8,000 barrels a day. Shell, which produces about half the oil from Nigeria, withdrew 50 workers from three facilities on Thursday, but output was maintained by a skeleton staff. The unrest adds to pressure on oil prices, already approaching historic highs because of Hurricane Katrina in the US.

"We will unleash upon the government and its cohorts violence and mayhem never before reported in the history of the Nigerian state," it said in a statement.

Mr Asari was arrested for alleged treason because he had called for the breakup of Nigeria in a newspaper interview. The Daily Independent quoted him as saying: "Nigeria is an evil entity ... I will continue to fight and try to see that Nigeria dissolves and disintegrates."

The group campaigns for self-determination for the Ijaw tribe, the largest in the delta, and argues that the colonial treaties that created the union with the rest of Nigeria are fraudulent. Despite its oil wealth, most inhabitants of the delta live in abject poverty. The resulting resentment fuels armed conflict, sabotage, kidnappings and theft from oil pipelines.

On Thursday a court in the capital, Abuja, granted a request by the justice minister to detain Mr Asari for two weeks to prepare charges of treason, which carries the death penalty.

Last year at least 2,000 people were killed in fighting between troops and Mr Asari's militia, until a peace deal was signed granting him amnesty in return for the militia's disarmament.