Two German oil workers kidnapped in Nigeria's Niger Delta area have been released, the German Foreign Ministry confirmed in Berlin Saturday. A spokesman said they were in good health but had been taken to hospital for precautionary tests, according to DPA. The two, employees of a Mannheim-based company sub-contracted by Shell, were seized on Wednesday with four Nigerian co-workers while travelling in a boat to a platform operated by the oil giant. The abductors had demanded a ransom of 20 million dollars, and that multinational oil concern Shell step up efforts to assist the poverty-stricken local population. A spokesperson for the ministry expressed "relief" at the news that the two had been freed and paid tribute to the "close cooperation" between German diplomats, the Nigerian authorities and the companies involved. In Mannheim, a spokesperson for the men's employers, building contractors Bilfiger Berger AG, said that no ransom had been paid. "The hostages are free and unhurt," said Sasha Bamberger. "We are happy and relieved." Kidnappings for ransom happen regularly in the Niger Delta, where the bulk of the population lives below the breadline. In extreme cases militant groups have occupied oil platforms to press their demands for a greater share of Nigeria's oil wealth. The situation is further complicated by tensions between rival gangs who illegally tap oil pipelines. Nigeria is Africa's biggest oil producer with a daily output of 2.3 million barrels.