The HTMS Sukhothai sank on December 18 roughly 37 kilometres (22 miles) off Thailand's southeastern coast, with a massive rescue operation managing to pull 76 crew alive from the waves.

"We found three bodies which will be brought into an identification and autopsy process, which will take about three or four days," said navy spokesperson Pogkrong Montradpalin.

He said in case of an oil leak, the area around the vessel and two others that sank that day had been declared a "disaster prevention zone".

The navy increased the death toll to 18 with one body found on Saturday confirmed as a crew member and a further two discovered, although Pogkrong did not give any further details.

Eleven members of the 105-strong crew are still missing, the navy said.

Helicopters, unmanned surveillance aircraft and warships have been combing the sea in hope of finding survivors.

On Tuesday, Admiral Chonlathis Navanugraha called the incident "one of the most severe tragedies" in the navy's history.

The vessel -- a corvette, among the smallest of military warships -- ran into trouble after its electronics system was damaged.

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