NAIROBI, Feb 12 (Reuters) - The U.S. Navy has fired on Somali pirates who hijacked the Russian crew and British captain of a ship sailing off the Horn of Africa, a maritime official said on Tuesday.

Four Russian crew members, an Irish chief engineer and a British captain were aboard the Svitzer Korsakov, an ice-class tug vessel, when it was seized early in February as it was making its way to Russia's Pacific Coast.

"A U.S. warship fired at one of the boats. We don't know if there are any casualties," said Andrew Mwangura, director of the Seafarers Assistance Programme.

The U.S. Navy declined to comment on reports that shots were fired.

"We have ships in the area monitoring the captured vessel, the Svitzer Korsakov," Navy spokeswoman Denise Garcia said.

"We have a full range of options available to deal with piracy and we will utilise them as the situation dictates," she said late on Monday.

Garcia said the navy was trying to stop the pirates from re-supplying.

Mwangura says the gunmen claim to be "eco-warriors" and not pirates.

Piracy has been rife in the waters off Somalia since warlords toppled military dictator Mohamed Siad Barre in 1991. But attacks have reached unprecedented levels due to instability onshore as the country's interim government battles insurgents.

The U.S. State Department calls it the "preferred venue" for pirate attacks in the region. (Additional reporting by Mohammed Abbas in Bahrain; Writing by Jack Kimball; Editing by Giles Elgood)

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