Nairobi - Pirates have seized a yacht and its four American occupants in the Indian Ocean off the coast of Oman, piracy watchdog ECOTERRA International said Saturday.

The yacht, the S/V Quest, belongs to American couple Jean and Scott Adam, who embarked on a round-the-world trip in 2004. It was not certain they were onboard. The possible identity of the other two American nationals was unknown.

“If the owners are on board, it would be a sad log for the couple on their 7-year world journey,” ECOTERRA International said.

US navy officials confirmed they were aware of the hijacking by Somali pirates, which occurred Friday, and said they monitoring the situation.

According to a website chronicling the Adams' voyage, they are travelling the world and handing out thousands of Bibles.

Pirates range far from Somalia in their search for vessels to hijack for ransom, defying a fleet of international warships sent to curb the ever-growing threat.

For the most part, ship owners give in to the multimillion-dollar ransom demands.

However, US Navy SEALs in 2009 shot dead three pirates who were holding the American captain of a commercial vessel hostage. The remaining pirate from that attack was this month sentenced to over 30 years in jail by a US court.

Somali has not had a functioning government since the 1991

ouster of dictator Mohamed Siad Barre, allowing pirates to set up base on land and flourish in the Horn of Africa nation's coastal waters.

ECOTERRA says 51 ships and two barges, with 819 crew, are being held hostage by Somali pirates, while the European Union's anti- piracy mission EU NAVFOR as of Friday counted at least 31 ships and 694 hostages. - Sapa-dpa