Devastating floods on the Nepal-Tibet border have killed at least 359 people, Nepali police said in an update on Thursday, a day after the disaster.

“As of 5pm [on] Thursday, 359 people have died,” Nepal Police said in a statement.

Chinese state media had earlier reported “major casualties” from a mudslide at a Tibet border hub with three dead, taking the total killed to at least 362.

A terrifying clip of surveillance video from the Chinese side of the frontier shows a massive wall of mud and debris crashing over a building several stories high on Wednesday morning, engulfing it as dozens of people ran for their lives below. The surge swept away buses and cars like they were toys.

Many of the missing are foreign tourists, officials said of the disaster, which the US Geological Survey believes was caused by a glacial collapse that sent a flow of debris engulfing areas along the way.

The tsunami-like rumble of water and mud struck villages and towns on the path of two rivers — Bhote Koshi and Trishuli.

When Nepal’s death toll stood at 273, 823 people, including 579 Nepali and foreign tourists, remained out of contact, according to the country’s disaster agency.

These included nearly 100 Chinese nationals, according to Beijing’s foreign ministry. The Chinese embassy was “seeking further verification” from Nepali authorities, ministry spokesman Lin Jian told reporters.

Later, about 100 missing Indian visitors and 25 Chinese workers were located by rescuers in Nepal, authorities said.

Across the border, Chinese state media said 558 people, including 260 foreigners, were still missing.

Meanwhile, the US State Department said 90 Americans were missing after the Nepal floods.

“We’re currently tracking 90 Americans who may be impacted by the flood, currently unaccounted for,” a spokesperson said.

The State Department said it could confirm that three Americans had been rescued and it was not aware of any US deaths.

Nepal’s Prime Minister Balendra Shah left by road for Bidur in one of the worst-affected districts of Nuwakot to assess the flood situation, his office said on social media.

Raju Bhadel, 56, a businessman in Nuwakot, recounted receiving a call from his brother-in-law in distress.

“They were crying and said their entire house had been swept away. Three family members were rescued, but his brother is still missing,” he said.

Also at Nuwakot, teacher Manok Mishra witnessed “nine trucks being swept away by the floods, including people”.

‘Inland tsunami’

Rescuers waded through mud and sludge to look for survivors after floodwaters buried the lower floors of multi-storey buildings.

Among those hunting for their missing family members was Subash Khatani, who told AFP he was looking for his big brother.

“He is among the missing from Rasuwa. That’s why we are out to search for him,” he 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.

Others included tourists from Malaysia, the Netherlands, Portugal, South Africa and South Korea, Nepal’s tourism board said.

Hiking company Himalayan Glacier told AFP that 47 people from a group, including two children, were missing in Nepal.

“Our teams are checking hospitals where victims are coming in,” said the company’s director Sanket Pandey.

The sudden rush of mud and debris was “like an inland tsunami, travelling nearly 170 kilometres down the Bhote Koshi valley into Central Nepal”, said environmental historian Ruth Gamble of La Trobe University.

“It is a popular route for international tourists travelling from Kathmandu to Lhasa,” she added.

“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 added. “She’s gone.”

Another survivor said he saved himself by clinging to a mango tree as he watched the house where he had been working disappear.

Rasuwa, in the north of Nepal, was the first district to be affected. But deaths have been reported from seven Nepali districts in total as the floodwaters raged through to Chitwan in the south, on the border with India.

“There is devastation everywhere we look,” said Tula Bahadur BK, a health worker in Rasuwa.

“The settlements next to the river have been completely swept away. Five of my own relatives are missing.”

The Nepali Army was carrying out flights and had rescued scores of people, while others living near the river in danger zones had been told to relocate.

China’s leader Xi Jinping said that “the most urgent task is doing everything possible to search for and rescue the missing people”, according to CCTV.

Tibet government officials mobilised nearly 800 emergency workers and 150 vehicles as well as sniffer dogs and speed boats to help the rescue efforts, China’s state-run news agency Xinhua said.

The International Federation of Red Cross and Red Crescent Societies (IFRC) said it was dispatching emergency supplies including blankets, first aid kits, stretchers and body bags.

“Entire villages and market areas have been wiped out,” said David Fisher, the Nepal head of IFRC.

The US State Department said it was sending a disaster response adviser and providing assistance of $500,000.

Australian Prime Minister Anthony Albanese said details on the disaster were scant.

“Nepal … is a developing country,” he said. “It doesn’t have the resources that an event like that would have in Australia, so information is difficult.”

UN teams and partners were sending supplies and personnel to support affected Nepali communities, Secretary-General Antonio Guterres said.

Glacial collapse

The cause of the disaster was not immediately clear in the hours after it occurred early on Wednesday, but the US Geological Survey said it was a glacial collapse that caused seismic activity and the “catastrophic” downstream impacts in Nepal and Tibet.

“This event was initially reported as a magnitude 4.4 earthquake,” it said.

“Additional analysis of nearby seismic stations, long-period seismic waves, and satellite imagery led to the determination that the seismic event was instead a glacial collapse and debris flow, and that no earthquake had occurred.”

Climate change is causing glaciers to melt worldwide, increasing the risks of glacial lake floods and glacial collapses.

Qianggong Zhang, head of climate and environmental risks at the Kathmandu-based International Centre for Integrated Mountain Development (ICIMOD), warned of the potential for a second flood.

“With a blockage still lodged upstream on the Nepal-China border river, authorities warn a second flood may occur,” he told AFP.

Many trekking companies were not running tours during the monsoon season, but there remain significant numbers of travellers, including pilgrims, in the region.

Experts cautioned that a lingering blockage in a river could produce an additional flood.

Among sites hit by the flood was Syabrubesi, the gateway to the popular trekking areas, as well as by pilgrims travelling to the base of Mount Kailash, a holy site for Buddhists and Hindus.

Traffic authorities in Tibet’s Shigatse said roads in the area were “at risk of mudslides and unsafe for passage”.

Buddhist spiritual leader the Dalai Lama said that he was praying for all those killed in devastating floods in Nepal and Tibet.

“I pray for all those who have lost their lives, and offer my sympathy and heartfelt condolences to the bereaved and to all those affected by this tragedy,” he said in a statement.