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 HTMS Sukhothai sank after water flooded its power controls on Sunday night.

On Monday, authorities said 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, with the operation continuing on Monday with air force assistance.

The navy 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, the 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 32km (20 miles) east of Bang Saphan, in the Prachuap Khiri Khan province, when it got caught in the storm on Sunday.

Dramatic pictures posted on the Thai Navy's Twitter account show the vessel listing onto its starboard side, and back-up rescue ships trying to find survivors in choppy waters.

Three naval ships and helicopters were sent to assist, but only the HTMS Kraburi reached the vessel before it sank.

The frigate picked up most of the Sukhothai's crew, the navy said. Sailors wearing life jackets were found in the water and in life rafts. The navy has disclosed scant detail on their condition. Local media published pictures showing medical personnel at the dock taking crew members off in stretchers.

Prime Minister Prayut Chan-o-Cha released a statement on Monday confirming officials were investigating the disaster.

"I am following the news closely - about five people are seriously injured," he said.

The HTMS Sukhothai was built for the Thai navy in the US in the mid-1980s.