Twenty Myanmar migrant workers died early Friday as fire tore through a bus taking them from a Thai border town to Bangkok, officials said.

It was not immediately clear how the blaze started on the bus -- one of four travelling together to the Thai capital.

But television footage showed a bus destroyed by fire which trapped many passengers inside.

"The death toll is now 20, with three people injured," Pollawat Sapsongsuk of the Tak Disaster Prevention and Mitigation centre told AFP.

Kittisak Boonchan, a rescue worker in Tak province, told reporters there were 47 people on the bus.

The accident took place at 1:25 am in the northwestern province of Tak which borders Myanmar, a source of much of Thailand's huge migrant labour force.

Low status and poorly paid, migrant workers often fall victim to safety lapses and exploitation in Thailand.

The kingdom has a poor road safety record on crashes with around 24,000 people perishing on its roads each year, according to the World Health Organization.

Last week 18 people died when a bus veered of the road and smashed into a tree in the northeast of the country.

The driver of that bus later confessed to being on drugs at the time of the crash.