A U.S. based oil firm has resumed full operations in Nigeria's oil rich Niger Delta, after militants responsible for an attack last week declared a cease-fire. The separatist group has not followed through on threats of widespread violence against oil installations following the arrest of their leader.

U.S. based Chevron oil company has reopened its facilities in Idama and Robertkiri in Nigeria's volatile Niger Delta. The company had closed the platforms last week following an attack by armed militants on the Idama flow station.

Other major oil companies restricted employee travel around the area's main city, Port Harcourt, after militants began making threats against their facilities.

Together the two Chevron facilities produce more than 25,000 barrels of crude oil every day.

The decision to reopen them comes as a statement sent to journalists signed by the outlawed Niger Delta's People's Volunteer Force declared a cease-fire late Monday.

The militants had earlier claimed responsibility for the attack on the Idama station.

The group threatened widespread violence against government facilities and foreign oil interests, after its leader Mujahid Dokubo-Asari was arrested last week. He is currently in custody in the capital, Abuja, facing treason charges stemming from remarks he made in a newspaper interview.

Mr. Asari has, in the past, called for the creation of an independent Niger Delta republic. During his first appearance before a judge, he attacked President Olusegun Obasanjo as a dictator and said his detention was illegal.

The Niger Delta is the center of Nigeria's oil industry. But disputes over oil revenues and environmental destruction have fueled regular militant uprisings against the federal government.

An energy analyst for the London based research firm Global Insight, Olivia Amaewhule, says Mr. Asari's group did not start out as a separatist movement.

"Generally, the militant movements have started because of some concern over revenue sharing in Nigeria," said Olivia Amaewhule. "This particular one started out as a gang supporting some politicians during elections. And they have grown bigger than their initial mandate."

Mr. Asari threatened to unleash, what he called, "a full scale war on foreign oil companies" during a spate of fighting last year. The situation was only calmed after he met personally with President Obasanjo. Neither the government nor the militants followed through on subsequent promises.

And Ms. Amaewhule says the Niger Delta People's Volunteer Force remains a threat in the region.

"There was a peace deal in the process of being brokered late last year between two local gangs, where they were supposed to have given up their arms," she said. "But that was disrupted. So, they still retain some of the arms that they had before the peace deal was brokered."

The judge hearing the case against Mr. Asari said last week that he would be held for two weeks and then charged with treason. If convicted, the separatist leader could face the death penalty.