MOGADISHU - Somali pirates holding two Britons captive in the Indian Ocean have transferred them to another vessel and said on Thursday that they planned to keep them onboard off the Somali mainland.

Paul and Rachel Chandler, both in their 50s, had left the Seychelles on their 38-foot yacht Lynn Rival and were believed to be sailing to Tanzania when they were hijacked on October 23.

After we understood the British navy might attack us, we took the hostages off the yacht into the Singaporean ship to bring them safely here, a pirate called Hassan told Reuters by telephone from the coastal town of Haradheere.

They are close to Haradheere. We will be holding them in the Singaporean ship along with that ship's crew. We decided not to take them to shore. They are exhausted and they need rest.

Hassan was referring to a Singaporean container ship, the Kota Wajar, which was seized by Somali gunmen earlier this month along with its 21 crew.

Earlier on Thursday, the European naval force said a Spanish military helicopter had spotted the hijacked yacht. A spokesman for the force said there was no information on the couple.

They were not on board the yacht, no one was, the spokesman said. That would indicate they have been transferred to another vessel. We do not have any reason to believe that they have been harmed.

The Seychelles coastguard sent aircraft to search for the Chandlers' yacht after receiving a distress signal on Friday, while naval forces from the NATO alliance, European Union and United States had also joined the search.

Pirates have plagued busy shipping lanes off the coast of Somalia for several years. Foreign warships from 16 nations are in the area to try and prevent hijacks, but the sea gangs are now hunting for ships far into the Indian Ocean.

The gangs -- some made up of former fisherman angered by the presence of foreign fishing fleets in Somali waters -- and their backers within Somalia and abroad have made tens of millions of dollars in ransoms.

Also on Thursday, pirates boarded a Thai-flagged fishing boat, the Thai Union 3, and were taking it to Somalia, the EU naval force said, adding that skiffs used by pirates had been spotted on board.

Asia's biggest canned tuna exporter, Thai Union Frozen Products, confirmed one of its ships had been taken by pirates. The company said it had 25 crew members on board, none of them Thais. It did not give their nationalities.

The hijacking of the Thai vessel took the number of ships being held by pirates on the Somali coastline to eight.

Foreign warships from 16 nations are patrolling the area to try and prevent hijacks, but the sea gangs now hunt for ships far into the Indian Ocean, as well as the strategic shipping lanes linking Europe to Asia through the Gulf of Aden.

While hundreds of sailors have been held captive by the Somali pirates over the past few years, most have been released unharmed -- once a ransom has been paid.

(Additional reporting by Peter Griffiths in London and Bangkok bureau; Writing by Daniel Wallis; Editing by Giles Elgood)