The six, including one woman, were abducted on June 3 from the Aluminium Smelter Company of Nigeria (ALSCON), which is controlled by Russia's United Company RUSAL.

"The six Russian hostages were released to the Rivers state government this evening. They are doing well," said state government spokesman Emma Okah.

The release leaves at least six foreigners still in the hands of armed groups in the vast wetlands region, which is home to Africa's largest oil industry.

Militants complaining of neglect and demanding control over the region's oil resources have stepped up a campaign of attacks and kidnappings against the Western-run oil industry since late 2005. This has forced thousands of foreign workers to flee and curbed oil output by about a fifth.

The violence has spiralled out of control, spawning a rise in kidnapping for ransom, gang violence and armed robbery which has nothing to do with the political struggle.

More than 200 expatriates have been kidnapped in the delta since the start of 2006 and almost all have been freed unharmed in exchange for money.

But abductions have subsided since the inauguration on May 29 of President Umaru Yar'Adua, who promised to give the region his urgent attention.

Many militants expect him to negotiate a better deal for the remote region of swamps and mangrove-lined creeks, where most people live in poverty despite the riches being pumped from their land.