Two Germans and four Nigerian oil workers were kidnapped by armed militants during a raid on a boat in Nigeria's southern oil-rich Delta region.

An official with the German firm Bilfinger Berger, Thomas Horbach, said the gunmen stopped the supply boat Wednesday as it sailed from Delta State to Bayelsa State to inspect an offshore oil field owned by Royal-Dutch Shell.

The German firm works as a sub-contractor for Shell.

Militant groups frequently attack oil operations in the Niger Delta to demand social services and better job opportunities from multinational companies.

Poor residents often complain they have been cheated out of the huge riches extracted from their tribal lands - where the bulk of Nigeria's 2.3 million barrels of petroleum are pumped daily.

Some information for this report provided by AFP and AP.