An overcrowded bus carrying Hindu pilgrims skidded off a slippery highway in southern Nepal on Sunday, killing at least 35 people, officials said.

Police official Gyan Bikram Shah said the bus was so packed that some people were riding on the roof.

The driver lost control on the rain-slicked road, and the bus plunged into a flooded irrigation canal, Shah said. Rescuers have recovered 35 bodies.

Most of those on board are believed to be Indian nationals from the neighboring Indian state of Uttar Pradesh state who were visiting Nepal on an annual pilgrimage to Hindu temples there. However, the victims' nationalities and identities were not yet clear because Indian nationals do not need to register when they cross into Nepal, the official said.

Shah said the death toll is likely to climb higher.

The driver is believed to have survived the crash, but may have fled later, he said.

The area is about 160 kilometres southwest of the capital, Katmandu.