Major oil companies operating in Nigeria's oil-rich southern region say they will not give in to threats of attacks on their facilities and employees by militias.

The threats are being issued by the leader of the Niger Delta People's Volunteer Force, Mujahid Dokubo-Asari, who says all foreigners should leave the Delta region by October 1 or risk attacks.

The group issued a statement accusing Nigeria's largest oil producer, Royal Dutch Shell, of collaborating with the federal government in what the group calls acts of genocide. The Nigerian government has been cracking down on oil thieves who have been illegally siphoning oil from pipelines in the region.

A spokesman for Royal Dutch Shell, Simon Buerk, says oil production is suffering due to the unrest and threats of attacks in the region.

"We have also, we don't talk in details about security measures, but amongst the things we've done, we have curtailed movement of staff and supplies on the creeks by boat," he said. "We have had a technical problem at one flow station, which produces about 28,000 barrels of oil per day and because we've restricted movement by water in the interest of safety, we are presently unable to get staff up to that particular flow station to rectify the problem."

Mr. Buerk says the Niger Delta region of Nigeria accounts for just over 10 percent of the company's total worldwide oil production. But he says Shell would not speculate on the possibility of pulling out of the region, or directly comment on the recent threats.

The violence in Nigeria, Africa's largest oil producing nation, has helped spur the recent increase in worldwide oil prices, which have hit record highs.

A spokesman from Rivers State, Emmanuel Okah says the Nigerian government will not allow the militia leader's threat to be carried out and urged foreign oil companies to remain calm and to continue production as usual.

An armed uprising last year in the region led a number of oil companies to temporarily shut down operations, and halted nearly 40 percent of Nigeria's oil output.