Shell's operations in the Niger Delta have been targeted before

The workers, two Germans and four Nigerians, are employed by oil services firm Bilfinger-Berger, a local contractor for Shell.

A group of gunmen abducted the workers as they were travelling by boat.

Kidnappings have happened before in the area, with foreign oil firms accused of reneging on agreements to invest in the local economy, communities and region.

In December of last year, villagers in the Delta region temporarily occupied three foreign-owned installations - two run by Shell and one operated by ChevronTexaco.

There have been other incidents since then, the latest ending on Wednesday when an armed gang released Tony Gouldbourne, the Liverpool man they had seized during the hijack of an oil platform off the West African coast.

No word

Nigeria is a member of the Organization of the Petroleum Exporting Countries and most of its crude comes from the Niger Delta area.

Many local people have been angered because they believe that they are not benefiting from oil that is being extracted.

Thomas Horbach, a spokesman for Bilfinger in the port city of Warri, said that the Bilfinger workers had been taken as they were travelling to one of Shell's offshore operations.

"We haven't heard any word from them," Mr Horbach said.

According to the Reuters news agency, a group called the Iduwini National Movement for Peace and Development has claimed responsibility for the kidnappings.

The group said it wants to put pressure on the Nigerian division of Shell, and that it was angry the firm had not delivered promised jobs and building projects, Reuters reported.