The bus was en route to Sandhikharka from Butwal of Rupandehi district when the fatal accident happened in the Arghakhanchi district of western Nepal. The incident site is about 7 kilometres far from the district headquarters.
At least five persons have been injured in the incident, Nepal News reported.
According to Assistant Chief District Officer Bhesh Raj Khanal, the death toll may rise as some injured passengers are in critical condition.
As many as 14 people, including driver and his helper, died on the spot while one person died while being rescued.
The injured passengers have been rushed to the district hospital in Sandhikharka for treatment.
A series of fatal accidents have claimed many lives in Nepal recently.
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Last Sunday, at least 39 pilgrims, including 34 Indians, were killed when an overcrowded bus skidded off the road and plunged into the Gandak Canal in southern Nepal.
While 17 passengers died when their bus fell into the swollen Trishuli River near Krishnabhir in central Nepal on Monday.
In another accident on Monday three people were killed and 40 others injured when a bus collided with another vehicle in Sunwal of Nawalparasi district.