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At least 20 people were killed and another 17 hurt when a bus drove off a highway in Nepal and plunged into a river on Friday, police said.

Police office said the bus rolled about 100 metres down the slope early Friday and crashed into the fast-flowing Trishuli river near Chandibhanjyang, about 120 kilometres west of the capital, Kathmandu.

Police and villagers were helping the injured and pulling the dead from wreckage, which was mostly submerged in the river.

Police are investigating the cause of the pre-dawn accident.

Road accidents in mountainous Nepal are generally blamed on poorly maintained vehicles and roads.

Earlier this month, an overcrowded bus veered off a mountain trail in eastern Nepal, killing at least 33 people and injuring 28 others.