KUALA LUMPUR (AFP) - Four people, including three foreigners, who turned up after disappearing for more than a week in the South China Sea, were found by chance aboard a trespassing Vietnamese trawler, Malaysia's coast guard has said.

Spanish nationals David Hernandes Gasulla and Martha Miguel, Hong Kong citizen Tommy Lam Wai Yin, and Malaysian Armilla Alihassan went missing in their small boat on May 2 while on an island-hopping excursion off the Malaysian state of Sabah on Borneo island.

Malaysia's coast guard and navy launched a search-and-rescue operation but hope faded as days went by and they were not found.

The quartet were unexpectedly discovered on Thursday (May 12) when a Vietnamese fishing trawler was intercepted in the disputed Spratly Islands near Layang-Layang atoll, a popular diving spot, for encroaching in Malaysia's territorial waters, coast guard First Admiral Zubil Mat Som told local media.

Through the trawler's crew, authorities learned that the four had been picked up by two other Vietnamese fishing boats, he said during a press briefing in Sabah's capital Kota Kinabalu late Thursday, The Star reported.

The other Vietnamese ships were then contacted by radio.

"We were shocked. The woman on the radio told us she was Armilla and it didn't hit me then that she was among the four missing people. It was only when she said she was with Tommy that it clicked," he said.

The foursome were said to be in good health, and photos posted on Twitter by Malaysia's coast guard showed them smiling as they arrived back in Kota Kinabalu on Friday morning.

First Admiral Zubil said he did not know why the group went missing or how they came to be aboard the Vietnamese boats. They were to be interviewed by authorities on Friday.

Malaysian fishermen had recovered an engine entangled in their net last weekend that was believed to be from the missing boat, prompting fears that it had sunk and the group had perished.