Bangkok - A Thai tour bus taking university students on a field trip overturned into a ditch Friday, killing at least 22 passengers and injuring about 30 others, police said.

The driver lost control of the vehicle when navigating a steep curve in Nadi district, Prachinburi province, 140 kilometres northeast of Bangkok, said senior sergeant Thiradej Virakul of Wang Daeng police station.

"We suspect the brakes didn't work," Thiradej told Deutsche Presse-Agentur dpa.

The bus, belonging to the Chatulong Tour Company, was taking students from Kaset Techno Khon Kaen University to Chantaburi province. (dpa)