The bus, from the northeastern province of Yasothon, caught fire on a major four-lane highway in the central province of Saraburi, about 105km north of the capital.

The fire caused the bus to veer off the road and crash down a hillside, said Lt-Colonel Samran Soreekul, of Muak Lek police station, from the accident site.

The bodies of those who died were burned beyond recognition, while dozens of other passengers were injured and sent to nearby hospitals, he said.

The cause of the fire was still unknown, he said, but police investigators found it started from one wheel and spread to the engine chamber, causing the brakes to malfunction.

Bus conductor Panya Sarakul, 33, said the driver stopped the double-decker bus at Pak Chong district in Nakhon Ratchasima to fix a malfunctioning brake before driving on. But when it reached a downhill stretch in Saraburi's Muak Lek district, a wheel caught fire and the blaze spread quickly to other parts of the bus. After the bus slide down the hill and came to a halt, passengers tried to get out - but those who could not escape the wreck in time were burned alive.