Nepali rescue teams on Saturday recovered the first body from around 50 people missing after monsoon rains triggered a landslide that swept two buses off a highway and into a river.

The force of Friday's landslide in central Chitwan district pushed the vehicles over concrete crash barriers and down a steep embankment, at least 30 metres from the road.

"One body has been found about 55 kilometres from the accident site," said police spokesman Kumar Neupane.

District official Khimananda Bhusal said that roughly 50 people remained unaccounted for, revising down the number of missing from the 63 reported by authorities on Friday.

"It is hard to confirm the total number because we don't know if the buses stopped to add or remove passengers along the way," he said.

Dozens of rescuers spent hours struggling to comb the raging Trishuli river with rafts, sensor equipment and dive teams to find any trace of the passengers or the vehicles.

Teams on Saturday also moved downstream in hope of locating the missing passengers.

Fierce currents made worse by this week's torrential downpours have hampered their efforts so far.

Chitwan district chief Indra Dev Yadav said that all authorities in the area have been instructed to stay on alert for any signs of the missing.

"The river is narrow here and very deep," he said. "The water level is high, its speed is high and its turbidity is also high."

The accident happened before dawn on Friday along the Narayanghat-Mugling highway, around 100 kilometres west of Kathmandu.

One bus was heading from the capital to Gaur in Rautahat district in southern Nepal, and the other was en route to Kathmandu from southern Birgunj.

A driver was killed in a separate accident on the same road after a boulder hit his bus. He died as he was being treated at a hospital. (AFP)