The navy is searching for survivors after the HTMS Sukhothai, carrying 106 crew, sank in a storm.
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The Thai Navy says 31 sailors are missing after a warship carrying over 100 crew capsized and sank during a storm in the Gulf of Thailand.
The HTMAS Sukhotai sank after water flooded its power controls on Sunday night.
Authorities said on Monday they had rescued 75 crew but 31 were still missing in rough seas.
“It’s been more than 12 hours but we will keep looking,” a navy spokesman told the BBC.
Search crews worked through the night to find survivors, and the operation was continuing on Monday, the navy said.
They also announced an investigation into the cause of the disaster.
“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.
Officials said the ship went down after it took on water, which flooded its hull and short-circuited its power room.
With the power lost, crew battled to retain control of the ship which listed on to its side before sinking around 23:30 local time Sunday (16:30 GMT).
The ship had been on a patrol just 32km (20 miles) west of Bang Saphan, in the Prachuap Khiri Khan province, when it got caught in the storm on Sunday.
Dramatic pictures posted by the Thai Navy’s Twitter account show the vessel listing on to its starboard side, and back-up rescue ships trying to find survivors in the choppy waters.
Three naval ships and helicopters were sent to assist, but only the frigate HTMAS Kraburi reached the vessel before it sank.
The frigate picked up the majority of the sailors on board the Sukhotai, the navy said. Life-jacketed sailors were found in the water in life boats.
The navy has disclosed scant detail on their condition. Local media published pictures showing medics at the dock taking crew members off in stretchers.
The HTMS Sukothai was built for the Thai navy in the US in the mid-1980s.