The disruptions in oil production caused by radical separatists escalated Friday as Chevron Corp. announced it had shut down a second oil flow station in the Niger Delta region and Royal Dutch Shell PLC evacuated personnel from two other facilities.

The developments came as the Niger Delta People's Volunteer Force issued a statement threatening to destroy the nation's oil industry -- the fifth-largest exporter to the United States -- unless Nigerian authorities freed the group's leader, Moujahid Dokubo-Asari.

Dokubo-Asari, 40, was arrested Tuesday on allegations of treason, a capital offense, after he called for the dissolution of Nigeria in a newspaper interview. He has been in custody since then and has been ordered detained for two more weeks until formal charges are brought.

Haz Iwendi, the national police spokesman, said both the military and police are on alert in the Niger Delta, with 900 extra officers entering the region in the past two days. "There is no reason for panic at all," Iwendi said in a brief interview. "Everything is calm."

A group calling itself the Supreme Council of the Niger Delta People's Volunteer Force said in a statement e-mailed to news organizations Thursday: "We will kill every iota of oil operations in the Niger Delta. We will destroy anything and everything. We will challenge our enemies in our territory and we shall feed them to the vultures."

The group's leaders have claimed that members, armed with only machetes and sticks of dynamite, have taken control of 10 flow stations. There are dozens of flow stations, which direct oil from wells into pipelines and then onto tankers for export, in the impoverished Niger Delta.

Nigerian authorities and oil industry officials have portrayed a far more limited picture of the disruptions. Edith Azinge, a spokeswoman for U.S.-based Chevron, reported that a group of young men confronted officials at one flow station Thursday, prompting the company to shut it down. She said a second flow station, the Robertkiri platform, was shut down Thursday because of reports that it would be targeted next.

There were no injuries, Azinge said, and the protesters are not occupying either facility. Together, the two flow stations produced 27,000 barrels of oil a day. Overall, Nigerian oil wells produce 2.4 million barrels a day.

"It's not too much of a problem," Azinge said by phone from Lagos. She said there was no schedule for resuming operations at the two flow stations.

Alali Horsfall, the second-in-command of Dokubo-Asari's militia, warned all foreign workers to leave in the Ijaw tribal area of the oil region, the Reuters new agency reported from Port Harcourt, a town in the delta.

Royal Dutch Shell, which produces nearly half of Nigeria's oil exports, announced on Friday that it was evacuating nonessential staff from two flow stations but that automated operations were continuing.

Violence and brinkmanship have long plagued Nigeria's oil industry, which produces billions of dollars for the government and foreign investors each year while providing little for residents, community leaders have said. Wells and flow stations pump oil from beneath villages that lack even clean water.

Dokubo-Asari has long advocated the secession of the delta, and it is not clear why his latest published comments prompted his arrest. He was quoted as saying, "Nigeria is an evil entity. . . . I will continue to fight and try to see that Nigeria dissolves and disintegrates."

Moujahid Dokubo-Asari

is in custody on treason charges.