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ABUJA, May 30 (Reuters) - Four American hostages taken from Nigeria's oil-producing Niger Delta region were released on Wednesday after three weeks in captivity, authorities said.

The four contractors were taken by gunmen from a barge operated by U.S. oil company Chevron <CVX.N> off the coast of Nigeria on May 9, one of a series of kidnappings of foreign workers in the world's eighth largest oil exporter.

"The four American hostages have been released. The militants have handed them over to us," said Felix Ofore, a spokesman for the Delta state government.

One of the hostages, Larry Plake, said they had no access to clean drinking water and were kept in filthy surroundings.

The release leaves about 20 foreign hostages still in the hands of various armed groups in the Niger Delta, a remote, wetlands region which is home to Africa's largest oil industry.

Unrest is fuelled by widespread resentment against an industry which has extracted billion of dollars of oil, but left the majority of people without basic services such as electricity, clean water, schools and hospitals.

Militants fighting for regional control over the delta's oil wealth have sparked an upsurge in attacks on the oil industry since early 2006, reducing Nigerian oil exports by a quarter.

But the line between militancy and crime are blurred and most kidnappings are by groups seeking ransoms.

The release of the four Americans, who work for U.S. contractor Global Industries, occurred on the same day that another armed group had promised to free another six foreign oil workers kidnapped on May 1.

The six men -- four Italians, an American and a Croat -- were taken from an offshore oil platform also operated by Chevron.

It was unclear what caused the delay in the release of the group of six.

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