More than 70 survivors from the HTMS Sukhothai have been hauled from the sea since the vessel went down late Sunday roughly 37 kilometres (22 miles) off Thailand's southeastern coast.

Rescuers in helicopters, two planes, and four vessels -- the HTMS Kraburi, HTMS Angthong, HTMS Naresuan and HTMS Bhumibol Adulyadej -- were scanning the turbulent waters for more than 20 sailors still missing, the navy said.

"Four found dead," commander-in-chief of the Royal Thai Navy Choengchai Chomchoengpaet told a press conference in Bangkok.

Earlier Tuesday, naval commander Pichai Lorchusakul confirmed rescuers had found, in good health, 23-year-old Chananyu Gansriya, from Loei province.

"I believe that this is good news that we could find more people," he said, adding that Chananyu was being cared for onboard the HTMS Angthong.

Efforts to find the missing crew were focused on aerial searches, with the Royal Thai airforce assisting the operation, which has been affected by strong winds.

- 'Hopeful' families -

At a pier in Prachuap Khiri Khan province, anxious families of those missing gathered to wait for news.

Siri Esa, the mother of 21-year-old Saharat Esa who was onboard, had a smile on her face when she heard about the latest rescue.

"I also have faith in my son. This is good news," she said.

Tuesday's waves were still high, navy spokesperson Admiral Pogkrong Montradpalin said, noting the search area had grown and was focusing "on the area near shores, according to the currents and the wind".

Another naval officer, Narong Khumburi, had earlier expressed hope that more survivors would be discovered, as they had life vests -- "But I imagine they must be exhausted."

Sahachart Limcharoenphakdee, a member of the National Institute for Emergency Medicine, said they were working with naval personnel to care for those plucked from the waters.

"I am hopeful, and have trust for the navy rescue team, who are skillful," he said of the search for survivors.

Local doctors have done health checks on those rescued earlier, with some moved to hospital.

Some "suffered from broken bones in the upper arm and fingers", Wara Selawattanakul, a doctor with the provincial health department said.

Mother Phongsri Suksawat, 50, said she hoped "100 percent" that her 22-year-old youngest son Chirawat Toophorm would come home.

"I thought it would be fine and nothing bad would happen from the storm," she said, adding that before he went on the ship her son asked her to care for his wife.

"I would like to hug him."

- Electrical fault -

The vessel -- a corvette, the smallest type of military warship -- is believed to have run into trouble after its electronics system was damaged, according to the navy.

Parts of southern Thailand have been hit by storms and flooding in recent days.

A warning from the Thai meteorological office remained in place Tuesday, with strong winds causing rough conditions in the Gulf of Thailand. Seafarers were cautioned to be careful, and small boats were advised to stay ashore.

The HTMS Sukhothai was commissioned in 1987 and built in the United States by the now-defunct Tacoma Boatbuilding Company, according to the US Naval Institute.