The HTMS Sukhothai capsized late on Sunday night as it was patrolling the Gulf of Thailand, about 37 kilometres (22 miles) off the nation's southeastern coast.

Some sailors survived by jumping into a life raft at night, according to images shared by the Royal Thai Navy, which said 75 people had been rescued.

However, 31 sailors were still missing on Monday afternoon, according to navy spokesperson Admiral Pogkrong Montradpalin.

The search and rescue operation involved two Seahawk helicopters, two frigates and one amphibious ship, according to a navy statement.

A statement from the Royal Thai Air Force later Monday said they were also assisting in the operation, without giving details.

Prime Minister Prayut Chan-o-Cha said the cause of the incident was being investigated.

"I am following the news closely, about five people are seriously injured," he added in a statement.

The navy later said the search was still ongoing over an area measuring around 20 miles by 15 miles, concentrating on a section of sea roughly 18 miles south of the Sukhothai's last position.

"The operation will also cover the areas where the latest crew members were found," the spokesperson said.

The vessel is believed to have run into trouble after its electronics system was damaged, according to the navy.

"The ship's operating systems stopped working, causing the ship to lose control," the spokesperson said.

One image shared by the navy showed the vessel capsized, while a video showed a rescue boat alongside the ship in rough waters.

Other images shared on the navy's Twitter account showed men wrapped in blankets following their rescue.

Some of those rescued were being airlifted to a hospital in Sattahip, while uninjured crew would be taken to a naval base.

- Storms -

In one clip captured by local media, an unnamed crew member said he had been forced to stay afloat for three hours before being rescued.

"The waves were quite high, about three metres when the ship sank," he said.

"I put on the life jacket and jumped. I swam for three hours," he added, describing how one of the rescue vessels could not get close enough because of the waves.

Several areas in southern Thailand have been hit by storms and flooding in recent days.

The Thai metrological office said Monday that strong winds were causing stormy conditions in the Gulf of Thailand, warning seafarers to proceed with caution and small boats 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.

In 2018, a boat carrying mostly Chinese tourists capsized off Phuket island on Thailand's west coast.

More than 40 people died in the accident, one of the worst boat disasters in the country's recent history.