LA PAZ, July 27 (Xinhua) -- At least 18 people were killed in less than 24 hours in two separate highway accidents in Bolivia, police said on Friday.

The first accident occurred Thursday night when two public transit minibuses collided head-on along a tract of the Potosi-Uyuni Highway in the northwest town of Chanca, in Bolivia's plateau region.

Nine people were killed and 13 others were injured, according to a police report.

The second accident occurred early Friday when a passenger bus crashed into a roadside ditch along the highway linking the departments of central Cochabamba and eastern Santa Cruz.

Nine people were killed in this accident, with six others injured.

Oscar Ayala, the police commander of Sacaba, where the crash took place, said the accident occurred when the bus fell into a ditch, resulting in eight people killed on the scene and a little girl dead at hospital.

Police speculated the accidents could have been caused by brake failure or speeding.