A bus with at least 40 people on board plunged off a Nepalese highway and into a river on Friday, police said, with most passengers rescued but the search still on for more survivors.

The bus was travelling from the tourist city of Pokhara to the capital Kathmandu when the accident occurred in the central district of Tanahun.

"We have rescued 31 passengers so far but 14 of them were unconscious," district police spokesman Deepak Kumar Rai told AFP.

"Many have been sent to the hospital but we do not have confirmation on deaths. We are searching for the missing."

A military helicopter was deployed to transport the rescued passengers, Nepal Army spokesman Gaurav Kumar KC told AFP.

The accident comes a month after two buses with 59 passengers on board were swept into a river by a landslide in the nearby district of Chitwan.

Three people were able to escape alive, but authorities have so far only managed to recover 20 bodies from that accident, and the search is continuing for the two buses and the missing passengers.

Deadly crashes are relatively common in the Himalayan republic because of poor roads, badly maintained vehicles and reckless driving.

Road travel becomes deadlier during the annual monsoon season as rains trigger landslides and floods across the mountainous nation.