Bangkok - Thai marine police on Monday were searching for six foreigners and one Thai who went missing after their speed boat sunk near the Similan Islands, a popular scuba-diving destination in the Andaman Sea.

"We're still searching for the missing," Marine Police Lieutenant Colonel Wanlop Phuangbaka said.

Among the missing were two Australians, two Swiss, one German, one Japanese and one Thai, Wanlop said in a telephone interview with Deutsche Presse-Agentur dpa.

Thai fishing boats Monday morning rescued 23 survivors, including 15 foreign tourists and eight Thai staff, who were found drifting at sea after their diving boat sank en route from the Similan Islands to Phuket Island, about 22 kilometres offshore of Phuket.

The dive boat, named Choke Somboon, sent out a distress message at 11 pm (1600 GMT) Sunday as it was returning to Phuket, one of Thailand's most famous beach resorts situated 600 kilometres south of Bangkok in the Andaman Sea.

Contact with the boat was lost thereafter, Wanlop said.

Dive Asia, the owner of the boat, asked that a search-and-rescue operation be launched Monday morning.

The Similan Islands, a national marine park, is one of Thailand's best-preserved scuba and snorkeling destinations. Hotels and guest houses are prohibited on the islands to conserve the environment. (dpa)