Nigeria is one of the world's largest oil exporters

The move follows violent clashes between Nigerian navy troops and Ijaw ethnic militants at the village of Okerenkoko on the Escravos River, which left five people dead.

The village is close to the oil port of Warri, where Shell and other multinational oil firms are based.

Shell's Nigerian subsidiary was evacuating employees "who are not vital to operations" from company facilities in the area, a spokesman said.

Elections approaching

Activists from the Federated Niger Delta Ijaw Communities accused the navy of provoking the hostilities by launching a premeditated attack on the village last Thursday.

The soldiers accused the community of planning to disrupt oil facilities and attack rival communities, the activists said in a statement.

The Ijaw claim their fishing communities have been polluted by the oil industry.

They also have a long-standing grievance over lack of compensation from the oil giants.

The approach of next month's national elections, the first since Nigeria's return to civilian rule, has also heightened tensions in the area.

A Shell spokesman said the dispute had nothing to do with the company's operations in Nigeria.

Shell, which produces about half of Nigeria's output of two million barrels a day, has key facilities in the area.

The company told BBC News Online that it had closed down two of the region's 40 flow stations - Jones Creek and Egwa 1 - which together pump more than 30,000 barrels per day of crude oil.

Marine operations on the two rivers in the region have also been suspended.

Nonessential staff have also been withdrawn from the Odidi 1 and Escravos flow stations, but the company says that production there is unaffected.