At least 29 people have died and four were badly injured after a bus plunged into a deep ravine in northeastern Thailand, police say.

The accident occurred around midnight on Friday in the Lom Sak district of Phetchabun province, with the bus en route to the northern province of Chiang Rai.

"We suspect the bus driver fell asleep," Major General Sukit Samana, police commander of Phetchabun province, told the AFP news agency. "We found 28 bodies in the ravine, which is about 30 metres deep. And one died at the hospital."

Four others were in critical condition.

Samana said an eyewitness reported seeing the bus driving "very fast" before it plunged into the ravine.

The bus, which was carrying 40 passengers, was completely destroyed.

Source: Agencies