By Jean-Marc Mojon

NAIROBI — Somali pirates on Friday hijacked a yacht with four Americans on board in the Indian Ocean, a non-governmental organisation monitoring regional maritime activity said.

Ecoterra International said the S/V Quest was seized in the afternoon 240 nautical miles (275 miles) off the coast of Oman.

"S/V Quest was attacked by pirates in the Indian Ocean and the four Americans on board are being held hostage," it said.

"The sailing yacht was reportedly now en route from India to Mina Raysut, the industrial port of Salalah, Oman," Ecoterra added.

The S/V Quest's owners, retired couple Jean and Scott Adam, have been sailing around the world for more than seven years and detailed in a December update on their website what their travel plans would be for 2011.

"Our ports of call will be: Galle, Sri Lanka; Cochin, India; Salalah, Oman; Djibouti, Djibouti; The Suez Canal; and Crete. That gets us to April," the website entry read.

Despite Somali piracy being at its highest ever levels, with more than 40 ships and 800 seamen currently held according to Ecoterra, the US couple made no reference to the threat on their website.

The yacht beacon's position shows Mumbai in India but it was last updated six days earlier and a distress signal was sent from the middle of the Indian Ocean on Friday.

Most of the ships currently held for ransom by Somalia's marauding pirates are merchant vessels but small yachts have been seized in the past.

A yacht was hijacked off the Seychelles in October. The skipper escaped and was rescued by a naval vessel but two South Africans remain hostage.

In November 2010, British couple Paul Rachel Chandler were released after an ordeal that last more than a year, in what became one of the longest and most high-profile hostage cases in Somalia's recent, troubled history.

They had sailed from the Seychelles straight into a dangerous area at the peak of the piracy season.

Two French yachts were also hijacked in 2008 and 2009.

When pirates capture an oil tanker or a large cargo ship, which are insured by large companies, their priority is to establish contact with the ship's owner to negotiate a ransom payment, paying little interest to the crew.

But when they seize a private yacht, the ship itself is of little value to them and their only bargaining chip becomes the crew's life and freedom.

Pirates have tended to abandon hijacked yachts and take crews onshore, where they can better shield their hostages and themselves from military intervention.

Few Americans have been taken hostage by pirates since attacks in the Gulf of Aden and Indian Ocean surged in 2007.

In one famous case in 2009, the US captain of the Maersk Alabama, Richard Phillips, gave himself up to the pirates to save his crew.

He was later rescued in an operation by elite US Navy SEAL commandos in which three pirates were killed. Phillips was given a hero's welcome when he returned to his home country.

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