Nearly 1,500 people are missing, the majority of them tourists, authorities have said, after a catastrophic flash flood hit Nepal and Tibet, roaring down valleys and wiping out entire villages, with the preliminary death toll standing at 165.

Rescuers are racing to locate people missing in the mountainous region popular with trekkers and pilgrims, including the hundreds of nationals from India, the US, UK, Australia and Canada.

Videos from the event on Wednesday showed a massive wall of water crashing down on the border as people attempted to flee. Trucks were upended and houses, roads ⁠and power projects swept away in Nepal, while ​officials in Tibet said they feared "major casualties" after a mudslide hit a border crossing.

In the affected areas, houses, trees and vehicles were submerged or buried in mudslides. Helicopters hovered overhead all day, helping to rescue survivors.

Almost 600 foreigners were among 826 people missing in Nepal, police and tourism ⁠officials said. Among those unaccounted were 177 from India, 63 from the US, 34 from Australia, 33 from Britain and 25 from Canada.

Chinese state media said at least 558 were missing on the Tibet side of the border, of whom at least 260 were foreign nationals.

Police in Nepal said 165 ⁠bodies had been recovered by late on Wednesday. Chinese state media reported three dead on the Tibet side of the border.

Many of the victims were on a pilgrimage to Kailash Mansarovar, a high-altitude site in ‌Tibet revered by Hindus and Buddhists among others.

Early reports from Nepali ​officials suggested an earthquake could have caused the lower part of a glacier to collapse and trigger ‌the floods. But the US Geological Survey later said what was initially reported ‌as a 4.4-magnitude earthquake was a seismic shock generated by glacial rock and ice collapsing and the flow of debris. "No earthquake had occurred."

An initial study of Planet Labs satellite imagery indicated that a landslide of ice and rock triggered the debris-laden flood in the Lhende River, about 12 miles (20km) north-east of the Nepal-China Rasuwagadhi border crossing, Nepal's national disaster authority said. Experts say the risk of such events is compounded by the extreme topography of the Himalayas.

People shared harrowing stories of how they managed to survive. "I started running. Then a wall of water came thundering down as if it was an earthquake," said Bibek Kumal, a labourer who was working downstream from the worst-hit Rasuwa district, near Nepal's Himalayan border with China's Tibet.

From his hospital bed, Kumal said the torrent swept him downstream amid mud and debris but he survived by clinging to a mango tree. "Had there been no mango tree, I would ‌have been washed away to death," he said.

Keshav Prasad Baral, 68-year-old survivor, who was being treated at a small hospital, said: "It was a huge gush of mud coming straight towards us. As it got closer, it kept rising higher and higher.

"I tried to grab my wife's hand and take ⁠her to the terrace. But she was swept away by the mud. Four or five hours later, a helicopter came to rescue me. My wife is still somewhere beneath the mud. She's gone."

David Fisher, the head of the Nepal delegation of the International Federation of the Red Cross, said "entire villages and market areas have been wiped out".

The United Nations secretary general, António Guterres, said UN teams and partners were mobilising supplies and personnel to support communities affected by the disaster. The US government said it would provide $500,000 in emergency assistance.

Tibet government officials deployed nearly 800 emergency workers and 150 vehicles as well as sniffer dogs and speed boats to help the rescue efforts, China's official Xinhua news agency said. The Chinese president, Xi Jinping, said: "The most urgent task is doing everything possible to search for and rescue the missing people."

People from the affected areas of Nuwakot and Dhading were seen gathering outside a Nepal army training centre – the base for helicopter rescue operations – seeking information about missing family members.

Police were recording the names of the missing just outside the barracks gate. Mobile phone torches were their only source of light as the area remained without electricity due to damaged hydroelectric infrastructure.

The tour company Himalayan Glacier said 47 people, including two children, were missing from one of its groups in Nepal. Among them were 15 Australians, 10 Canadians, nine Nepalis, eight Americans, two Britons, two Singaporeans and one Russian-American dual citizen, its managing director, Sanket Pandey, said.

The Australian prime minister, Anthony Albanese, said his thoughts were with those concerned about their loved ones, and that getting information about the status of missing people remained difficult.