A bus crashed and killed 18 people in Thailand on Wednesday after the brakes apparently failed as it went down a hill, police said.

The vehicle was one of three double-decker buses carrying local government officials on a field trip when it veered off the road in Prachin Buri province, east of the capital Bangkok.

"There were 17 people found dead at the scene and one more later died in hospital," said Police Lieutenant Colonel Siwapass Phuripatchaiboonchu.

"We have questioned the driver and he said the brakes failed as the bus went down the hill."

A total of 49 people were on board when the bus crashed, and the survivors have been taken for treatment to two hospitals.

Thailand has one of the worst road safety records in the world, with around 20,000 people killed every year according to the World Health Organization.

Unsafe vehicles and poor driving are major factors contributing to the high toll.

In October, a school bus crashed and caught fire in a Bangkok suburb, killing 23 people — most of them children — in the kingdom's deadliest road accident in a decade.