A landslide swept two passenger buses carrying at least 63 people into a swollen river in central Nepal early Friday while continuous rain and more landslides were making rescue efforts difficult.

Three survivors apparently swam to safety, but rescuers by late morning had not found any trace of the buses, which likely were submerged and swept downstream in the Trishuli River.

Heavy monsoon downpours in the past few days have swollen the waterways and turned their waters murky brown, making it even more difficult to see the wreckage.

The buses were on the key highway connecting the capital to southern parts of Nepal when they were swept away around 3am near Simaltal, about 120 kilometres west of Kathmandu.

More landslides blocked routes to the area in several places, officials said. Additional rescuers and security forces were sent to help with the rescue efforts. Police and army personnel were searching using rubber rafts. Divers with scuba gear were also dispatched, according to the Chitwan district police.

One bus was carrying at least 24 people, while the other had at least 42, but more could have boarded en route, officials said.

A third bus was hit by another landslide on Friday morning a short distance away on the same highway. Officials said the driver was killed but it was not clear if there were any other casualties.

Nepal's Prime Minister Pushpa Kamal Dahal said he was saddened by the news and expressed concern over recent flooding and landslides.

He added that several government agencies were searching for the missing, in a post on the social media platform X. (AP)