108 people were injured but only 39 were kept for treatment. (Representational)

An overloaded bus plunged down a hill in central Nepal, killing at least 11 and injuring 108 people, an official said Saturday.The packed bus was ferrying passengers -- who had been celebrating the Hindu festival of Dashain -- from Sindhupal chowk to neighbouring Kathmandu.

But the crowded vehicle slipped and fell more than 50 metres at a bend.

"Six people were killed instantly and five more passed away on the way to hospital or while being treated," district official Goma Devi Chemjong told AFP. Ms Chemjong said 108 people were injured but only 39 were kept for treatment. "We are investigating what caused the accident," she said.

Accidents are common on Nepal's poorly maintained highways, with the number increasing during the festival season when the roads are far busier and public transport overcrowded.