"The bus carrying 34 people fell 150 metres down the highway, killing four on the spot," Birendra Bishwokarma, spokesperson for district police.

He noted that three of the injured were in serious conditions, adding overspeed may be the cause of the fatal accident, Xinhua news agency reported.

In a separate accident on Friday morning, two passenger buses collided in Nepal's Chitwan district, leaving five dead and five others injured.

Hundreds of people lose their lives in traffic accidents in Nepal each year.