Rescuers in Nepal used helicopters to search for survivors and drop emergency supplies to thousands cut off in towns and valleys after Wednesday's disaster swept a road thronged by trekkers and pilgrims that links Kathmandu with Lhasa in Tibet.
The death toll in Nepal was updated to 270 on Thursday, a figure that is expected to continue rising.
"This will go up, as we have resumed the search and more bodies are expected to be recovered," police spokesperson Abi Narayan Kafle told Reuters.
Almost 500 foreigners were among 826 people missing in Nepal, police and tourism officials said.
Among the missing tourists from more than two-dozen countries, 177 were from India, 63 were from the United States, 34 from Australia, 33 from Britain and 25 from Canada.
Malaysia's foreign ministry said 55 nationals were uncontactable.
The tally of missing in Tibet's Gyirong county was 558, Chinese state broadcaster CCTV said on Thursday, including 260 foreign nationals.
China said the death toll stayed at three.
Verified videos showed crushing masses of water and debris sweeping away dozens of people at a border crossing in Gyirong, with homes, roads and power projects washed away in Nepal as the flows travelled downstream.
"It was a huge gush of mud coming straight towards us," said Keshav Prasad Baral, a 68-year-old survivor being treated in a Nepali hospital.
"As it got closer, it kept rising higher and higher. When I saw it enter our home, I tried to grab my wife's hand and take her to the terrace. But she was swept away by the mud."
A helicopter ferried him to safety four hours later.
"My wife is still somewhere beneath the mud," he said. "She's gone."
In the hardest-hit districts, helicopters are dropping tents, food and other emergency supplies provided by neighbouring India, said Raja Ram Basnet, a spokesman for Nepal's army.
More than 5000 army and police rescue personnel joined the rescue effort that retrieved 125 people on Thursday, including 20 foreigners, he said.
Nepali Prime Minister Balendra Shah left by road to assess the flood situation in one of the worst-affected districts of Nuwakot, his office said on social media.
Early reports from Nepali officials suggested an earthquake in the area could have caused the lower part of a glacier to collapse and trigger the floods.
The US Geological Survey said an earthquake with magnitude initially reported as 4.4 was in fact seismic energy generated by the collapse of glacial rock and ice, followed by debris flow.
Chinese authorities have identified risks from a still-dammed lake in Gyirong, upstream from the mudslide and flood-stricken Tibet-Nepal border crossing that could complicate rescue efforts, particularly as rains are forecast, CCTV said.
Many of the victims were making a pilgrimage to Kailash Mansarovar, a high-altitude site in Tibet revered by Hindus and Buddhists among others.
UN teams and partners were sending supplies and personnel to support affected Nepali communities, Secretary-General Antonio Guterres said.
Security footage captured on the Chinese side of the border showed many people fleeing from a torrent of rock, mud and water that destroyed buildings and vehicles.
The Gyirong border port has a key role in the flagship Belt and Road network linking China to South Asia, with logistics parks, yards, and parking lots built in the past decade as part of extensive development efforts.
In the affected areas, layers of mud metres thick swathed homes, trees and vehicles.
Helicopters ferrying survivors hovered overhead all day.
An initial study of Planet Labs satellite imagery showed a landslide of ice and rock unleashed a flood carrying debris in the Lhende River, about 20km northeast of the Nepal-China Rasuwagadhi border crossing, Nepal disaster management officials said on X.
