Oil companies in Nigeria's volatile Niger Delta are closing facilities and evacuating staff a day after armed militants shut down an installation owned by a U.S.-based firm. Fighters protesting the arrest of their leader say more attacks will follow, if he is not released.

Members of the Niger Delta People's Volunteer Force have set a Saturday deadline for the release of their leader before the group unleashes, what it called in a statement, "violence and mayhem never before reported in the history of the Nigerian state."

They are warning large oil companies to evacuate staff and say they are preparing to destroy petroleum facilities if their demand is not met. Similar threats made in the past have not been carried out.

Mujahid Dokubo-Asaris looks on as he appears before a court in Abuja, Nigeria, Sept. 22, 2005

The groups jailed leader, Mujahid Dokubo-Asari, was arrested Tuesday and is currently being held in the capital Abuja. A judge said he would remain in custody for two weeks while treason charges are prepared against him.

The charges stem from comments Mr. Asari made in a newspaper interview.

Militants followed through on threats to attack oil installations Thursday, forcibly shutting down a flow station in Idama operated by oil giant Chevron. The firm later closed another installation in Robertkiri after receiving information it was under immediate threat.

A representative of the Anglo-Dutch company Royal Dutch Shell denied earlier claims made by a NDPVF commander that militants had overrun four of its platforms. But he said, Shell was taking precautionary measures including restricting travel by its employees to vulnerable areas.

However, the company says its production has not been affected, as many of its facilities can operate without staff.

An energy expert with the London-based analysis firm Global Insight, Olivia Amaewhule, says there is little immediate risk to production, but there will likely be an eventual progressive effect.

"I think what Shell is doing is to pull out non-essential staff," she said. "Oil production will still continue, but it just means that if [there are] maintenance issues, they wont be addressed immediately. And this could cause some production to go offline."

Last year, Mr. Asari's group helped send oil prices on the world market to what were then record levels, after declaring, what he called, a full-scale war on foreign oil companies. The tensions eased after Mr. Asari met personally with President Olusegun Obasanjo. However, the separatist leader never followed through on a promise to disarm his followers, and the government failed to deliver on its own promises of more help for the youth of the Niger Delta.

Ms. Amaewhule says, though the current situation has yet to reach the scale of last year's crisis, in which militias fought openly with federal troops, she expects oil firms to further limit their activities.

"When the threats are announced, the people's lives are in danger, and those oil companies have a duty to withdraw their workers from dangerous situations," she said. "Now there will probably be more closures, not because there have been armed attacks, but as a precautionary measure."

Activists from the Niger Delta's dominant Ijaw community have long asked for a greater share of revenues coming from the oil wealth in their region. They blame foreign oil companies for widespread economic and environmental destruction as well.

The companies say they give back to communities by developing the infrastructure.