Fifty four illegal Myanmar immigrants died of suffocation while being smuggled into Thailand in a freezer truck near the Thai-Myanmar border, a police officer said on Thursday.

The police officer in the western coastal Thai province of Ranong told Reuters another 67 Myanmar citizens in the same vehicle were rescued with over 20 being treated in hospital.

"They were kept inside the sealed truck for hours without air because the air-conditioning system failed. Many of them pounded the sides of the truck for help," Sergeant Phuvanai Wattanasamai said.

The driver of the truck parked the vehicle on a road about five to six kms from the coast, let his passengers out, and fled the scene before midnight on Wednesday, Phuvanai said.

"We don't know how long they had been hidden inside the truck, but over 100 were packed into the vehicle," he said.

Eighteen of the dead were women.

An estimated one million people from neighbouring army-ruled Myanmar work in Thailand, mostly as illegal workers in factories, restaurants, at petrol pumps, and as crew on fishing trawlers.