Authorities 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.
Local media put the death toll at 359, with dozens of bodies swept down into the plains from where they were being recovered.
Nepal's Home Minister Sudhan Gurung said the focus remained on finding survivors as rescue teams found 100 missing Indian visitors, mostly pilgrims, and 25 Chinese workers in the worst-hit region around Trishuli.
More than 650 foreigners were still unaccounted for while Nepali authorities were yet to determine the number of locals missing.
The Red Cross said that entire villages had been destroyed and impassable roads and bridges had left communities cut off.
The full scale of the disaster remained unclear, it said.
"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."
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 remained at three.
The missing foreigners in Nepal belonged to more than two-dozen countries, including 288 from India, 63 from the United States, 51 from Malaysia, 34 from Australia, 33 from Britain and 25 from Canada.
China was in close communication with Nepal on search-and-rescue efforts, with nearly 100 Chinese nationals in the affected area across the border in Nepal, the Chinese foreign ministry said.
China's Premier Li Qiang, the country's number two official, arrived at the disaster-stricken site in Gyirong on Thursday afternoon, state-run Xinhua news agency said.
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.
Chinese government geologist Guo Zhaocheng said the ice-avalanche debris flow that caused the mudslide travelled about 50 metres a second, leaving people little time to react, and extended for more than 20km, causing severe losses.
Guo said a high-altitude ice avalanche triggered the debris flow, which picked up a glacial moraine as it moved down a channel before becoming a mudslide that struck port facilities and areas on both sides of the China-Nepal border.
Ganesh Aryal, the administrator of Chitwan district, said authorities had recovered 103 bodies from the river system in his district which is about 100km away in the plains from the worst-hit region of Trishuli.
In the hardest-hit districts, helicopters were dropping tents, food and other emergency supplies provided by neighbouring India, Nepal's army 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.
Chinese authorities had 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 because rain were forecast, CCTV said.
On Thursday morning, the dammed lake was already overflowing, state media said, and more water was expected to enter the lake during the next three days, posing a high risk of a breach, with peak flow expected about September 1, it said.
Many of this week's victims were pilgrims headed to Kailash Mansarovar, a high-altitude site in Tibet revered by Hindus and Buddhists, among others.
Countries and organisations rushed or pledged help, with India, the United States, Britain, Canada and the United Nations saying they were providing assistance.
