NAIROBI, Kenya (AP) - The British navy on Thursday found an empty yacht in international waters belonging to a missing British couple and a defense official said Somali pirates may have transferred them to another vessel.

International naval forces have been searching for the couple for days. Paul and Rachel Chandler were heading to Tanzania in their yacht, the Lynn Rival, when a distress signal was sent last Friday.

A Defense Ministry spokesman in London, who spoke on condition of anonymity in line with government policy, said that pirates may have transferred the couple to another vessel.

"Paul and Rachel Chandler were not on board the yacht and we do not have any reason to believe they have been harmed," a Defense Ministry statement in London said. "Royal naval vessels operating with our international partners under EU, NATO and combined maritime forces will continue to play a full role in efforts to secure Paul and Rachel's release."

A picture of the Lynn Rival on the Web site of the European Union Naval Force showed a white vessel rigged with two white sails. Pirates have claimed the couple would be taken to the Somali pirate town of Haradhere.

Relatives of the British couple have pleaded with their possible captors to end the family's "bad dream."

"If I was to give a message to the pirates, I'd say you've got the wrong people," said Jill Marshment, the sister of Paul Chandler, Britain's Press Association reported.

The discovery of the empty yacht came the same day that pirates hijacked a Thai fishing vessel north of the Seychelles islands, the European Union Naval Force said.

The Thai Union 3 reported it was under attack by pirates in two skiffs 200 miles (320 kilometers) north of the Seychelles and 650 miles (1050 kilometers) off the Somali coast, according to a press release issued by the headquarters for the EU's Operation Atalanta.

A naval aircraft sent to the scene saw pirates aboard the vessel and two skiffs tied up behind it. The EU force said the ship is now heading toward Somalia.

The latest seizure means pirates are now holding a total of eight ships, four of which were seized in the past two weeks.

Somalia has not had a functioning government for 18 years. The multimillion dollar ransoms the pirates collect are a strong lure for young gunmen in a country where nearly half the population is dependent on aid.

The high-seas hijackings have persisted despite an international armada of warships deployed by the United States, the European Union, NATO, Japan, South Korea and China to patrol the region.

Associated Press Writer Jill Lawless in London contributed to this report.