The death toll from last week's devastating flash floods in Nepal has risen to 903, disaster management authorities said on Monday, bringing the confirmed death toll in Nepal and China to at least 919.

The number of people missing in Nepal also rose sharply to 4,247, including 592 foreigners, while 546 people remain unaccounted for in China.

Thousands of rescue workers continued searching for victims and recovering bodies in affected areas of Nepal and south-western Tibet in China.

Many bodies have been recovered in southern Nepal, dozens of kilometres from the areas worst hit by Wednesday's floods, according to Nepalese authorities.

The first victims are now being buried in mass graves in Nepal, local media reported, a decision that has sparked controversy in the predominantly Hindu country, where cremation is customary.

Authorities said DNA samples, photographs and other identifying information were being collected before burial so that relatives could later identify their loved ones.

"Our absolute priority is returning every victim to their families," the Kathmandu Post quoted a police spokesman as saying.

However, where visual identification was no longer possible and remains were rapidly decomposing, authorities had to secure DNA profiles before carrying out a dignified burial, he said.

Meanwhile, rescue workers in Nepal are focusing on several hydropower plants that were badly damaged by mud and debris. Hundreds of workers are believed to be trapped in multiple tunnels at the sites in the Himalayan mountain range. Operations have been hampered by rising water levels and bad weather.

In the disaster-hit area of China, teams are trying to reopen a key access road to allow heavy equipment to be brought in. However, the risk of further landslides and flooding is hampering the effort. More rain is forecast in the region in the coming days.