A yacht with four Americans on board is believed to have been hijacked in the Arabian Sea, the US embassy in Nairobi said on Saturday.

Pirate gangs plaguing the shipping lanes through the Gulf of Aden and Indian Ocean typically target large merchant ships, with oil tankers the prize catch, but the snatching of foreigners can also yield high ransoms.

"Preliminary reports indicate that a US-flagged vessel tentatively named as the Quest has been hijacked in the Arabian Sea. There were four US citizens on board," an embassy spokesman said.

All relevant US government agencies were monitoring the situation, he added.

Earlier, a regional maritime expert said the 58-foot S/V Quest had been hijacked 240 miles off Oman on Friday afternoon as it sailed from India to Salalah in Oman.

Ecoterra, an advocacy group monitoring piracy in the Indian Ocean, said the 58-foot yacht was owned by Jean and Scott Adam. It was not immediately clear if the couple were on the yacht at the time of the attack.

The couple began a round-the-world trip in 2004, according to their website.

East Africa maritime expert Andrew Mwangura said the ship was now heading towards Somalia, on the Horn of Africa.

Somalia has been mired in violence and awash with weapons since the overthrow of a dictator in 1991, and the lack of effective government has allowed piracy to flourish.

Pirate gangs in the Indian Ocean are making tens of millions of dollars in ransoms, and international navies have struggled to contain the problem owing to the vast distances involved.

Pirates in southern Somalia are still holding two South Africans seized from their yacht late last year. In November, another gang released the British couple Paul and Rachel Chandler after holding them captive for more than 14 months.

One French hostage was killed and four were freed in April 2009 when French forces attacked a yacht that had been seized by Somali pirates.-Reuters