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 believe was caused by a glacial collapse that sent a flow of debris engulfing homes 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, with Nepali police saying they had recovered 162 bodies.

Another 823 people, including 579 Nepali and foreign tourists, remained out of contact in Nepal, according to the country's disaster agency.

Across the border, Chinese state media said the "mudslide disaster" at the Gyirong Port in Tibet claimed three lives while 558 people, including 260 foreigners, were still missing.

Raju Bhadel, 56, a businessman in Nepal's Nuwakot district, 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.

The foreigners out of contact included Indians, and others from Australia, Britain, Malaysia, the Netherlands, Portugal, South Africa, South Korea and the United States, Nepal's tourism board said.

Hiking company Himalayan Glacier told AFP 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 in Australia.

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

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 flood waters raged through to Chitwan in the south, on the border with India.

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.

- Glacial collapse -

The cause of the disaster was not immediately clear in the hours after it occurred early 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 remains significant numbers of travellers including pilgrims in the region.

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 the 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".

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