A warship carrying more than 100 crew members collapsed and sank in the Gulf of Thailand during a storm, according to the Thai navy, leaving 31 servicemen unaccounted for.
On Sunday night, the HTMAS Sukhothai sank after water filled its power controls.
Authorities reported on Monday that they had saved 75 crew members, but 31 remained missing in high waves.
A naval official told the BBC, "It's been more than 12 hours, but we will keep looking."
The search for survivors continued on Monday with air force support after search teams worked all night.
The navy also said that it will look into what caused the accident.
“This has almost never happened in our force’s history, especially to a ship that is still in active use,” spokesman Admiral Pogkrong Monthardpalin told the BBC.
According to officials, the ship sank after absorbing water, which swamped its hull and shut off its power room.
The crew struggled to maintain control of the ship when the power went out, and it leaned to one side before sinking at 23:30 local time on Sunday (16:30 GMT).
The ship was trapped in the storm on Sunday as it was on a patrol 32 kilometres (20 miles) east of Bang Saphan in the province of Prachuap Khiri Khan.
Dramatic images of the ship listing to its starboard side and backup rescue ships searching for survivors in turbulent waters were uploaded on the Thai Navy's Twitter account.
Only the HTMAS Kraburi managed to reach the ship before it sunk despite the fact that three navy ships and helicopters were dispatched to help.
According to the navy, the frigate took up the majority of the Sukhothai's crew. In the ocean and in lifeboats, sailors wearing life jackets were discovered.
On their condition, the navy has provided very little information. Pictures of medical professionals at the dock escorting crew members off in stretchers were released in the local media.
Authorities were looking into the incident, according to a statement made public on Monday by Prime Minister Prayut Chan-o-Cha.
“I am following the news closely – about five people are seriously injured,” he said.
In the middle of the 1980s, the US produced the HTMS Sukhothai for the Thai navy.