The incident brings to nine the number of foreign workers being held hostage in the world's eighth largest oil exporter.

There have been a spate of attacks and kidnappings against foreign workers in the Niger Delta, Nigeria's main oil region.

   

A sick Italian oil worker who had been kidnapped in the remote creeks in another part of the Niger Delta on December 7 was set free on Friday. But the abductors said they would keep three other foreigners.