The 29 seat bus, carrying 19 passengers from a charity group, was travelling downhill when it flipped and was crushed, the Vietnam News Agency reported.
Rescue teams managed to pull 10 survivors from the wreckage, while several others remained trapped as of 10:30 a.m. local time. Heavy machinery was deployed to cut through the damaged vehicle, Hoang Anh Tuan, chairman of Phinh Ho commune, told VNA.
Tuan said preliminary information suggested the crash may have been caused by brake failure.
Police, military personnel, local officials and residents were mobilised for rescue operations. Authorities said an investigation into the cause of the accident is under way.
Attempts to contact officials in Yen Bai province and Phinh Ho commune for further comment were unsuccessful.
