At least 63 people were missing in Nepal on Friday after a landslide triggered by heavy monsoon rains swept two buses off a highway and plunged them into a river, authorities said.

Dozens of search and rescue personnel were combing the site for survivors of the accident along a remote stretch of road in the central district of Chitwan.

District official Khimananda Bhusal told AFP that the buses were carrying at least 66 people between them but three passengers had been able to escape as they crashed into the Trishuli river and were now being treated in hospital.

"We are not sure of the total number because the buses could have picked up others on the road," he said.

"The river has swollen and no one else has been found yet," he added.

Bhusal said that the survivors were out of danger and one had been discharged from the hospital.

One of the survivors told AFP from his hospital bed that his two children and two grandchildren had been aboard one of the buses when it hit the waterway.

"My son, my daughter, my grandchildren, all four are gone... only I am here," Jugeshwor Ray Yadav, 45, told AFP.

"I gulped down some water, but somehow I swam. I swam and then caught a branch on the hill," he said.

The force of the landslide pushed the buses over concrete crash barriers and down a steep embankment into the river, at least 30 metres (100 feet) from the road.

Search and rescue teams on rafts gathered on the riverbank were struggling to scour the muddy waters due to fierce currents made worse by the rains.

Hours after the search began, they had yet to find any trace of the vehicles or their remaining occupants.

"The teams are trying but river's flow is very strong. They have not found anything yet," police spokesman Kumar Neupane told AFP.

The accident took place before dawn along the Narayanghat-Mugling highway, around 100 kilometres (60 miles) west of the capital Kathmandu.

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

- 'Deeply saddened' -

Prime Minister Pushpa Kamal Dahal said he was "deeply saddened" by the accident in a post on social media platform X.

"I direct all agencies of the government, including the home administration, to search for and effectively rescue the passengers," he said.

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

Nearly 2,400 people lost their lives on Nepal's roads in the 12 months to April, according to government figures.

Twelve people were killed and 24 were injured in an accident in January when a bus heading to Kathmandu from Nepalgunj fell into a river.

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

Monsoon rains across South Asia from June to September offer respite from the summer heat and are crucial to replenishing water supplies, but also bring widespread death and destruction.

The rainfall is hard to forecast and varies considerably, but scientists say climate change is making the monsoon stronger and more erratic.

Floods, landslides and lightning strikes have killed 88 people across the country since the monsoon began in June, according to police figures.