YENAGOA, Nigeria (Reuters) - Six foreign sailors were freed unharmed overnight by pirates who seized them off the coast of Nigeria's oil-producing region nine days ago, police said on Tuesday.

A rise in piracy off Nigeria's oil-rich coast this month has shown gangs are willing to venture further afield and use more violent tactics, increasing the risk of doing business in Africa's largest energy producer.

One Russian, three Ukrainian and two Indian sailors were taken when gunmen stormed the Armada Tuah 40 miles (65 km) off the coast of Bayelsa state on February 17.

The ship is owned by Nigerian firm Century Group.

The pirates demanded a 200 million naira (859 thousand pounds) ransom last week but Bayelsa Police Chief Kingsley Omire told Reuters no money was paid for their release.

Exxon Mobil and Shell officials said last week that security was a major factor in making Nigeria one of the most expensive oil-producing countries to operate in.

Oil and shipping companies have to hire crisis management teams, pay huge insurance premiums and possibly ransoms, as well as face possible damage to their reputations.

Six pirates armed with automatic rifles fired on a cargo ship around 30 miles off the coast of the Niger Delta on Friday, according to piracy watchdog the International Maritime Bureau.

This is at least the sixth attack off Nigeria's coast this month, making it the biggest upsurge in piracy since off Africa's most populous nation since 2010, risk experts say.

(Reporting by Tife Owolabi; Writing by Joe Brock; Editing by Jon Hemming)

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