Passengers said the coach had swerved several times on winding mountain roads, before it ploughed through a fence and down a steep ravine, according to local police in Phitsanulok province 380 kilometers (235 miles) from Bangkok.
“Five people were killed – two men and three women, including one woman from Belgium- and a seven-month-old baby boy,” district police captain Sane Promrut told AFP on by telephone.
He said the bus, which was travelling from northeastern Udonthani province to the main northern city of Chiang Mai, plunged about 20 meters (66 feet) in the early morning crash.
Police said the dead Belgian woman was in her early twenties.
Hospital staff said the mother of the baby boy was in a critical condition, with internal bleeding, while the child’s father had suffered broken legs in the accident.
A 30-year-old Belgian man and a Japanese man were also among the injured.
“The bus had problems with its brakes and was speeding before it crashed over the cliff,” Sane said, adding that passengers had smelled burning and the driver, who is also in a critical condition, had stopped to try and fix the problem before the crash.