A deadly flash flood near Nepal's border with Tibet may have been triggered when the lower part of a glacier broke off and crashed on to the valley floor hundreds of metres below, experts said citing satellite imagery from Planet Labs.

It was not clear what caused the glacier to collapse, but Nepali officials have pointed to an earthquake in the region as a possibility. The US Geological Survey later said the reported tremor was in fact seismic energy generated by the collapse of glacial rock and ice, followed by debris flow.

Authorities on both sides fear the total number of casualties will be far higher than the nearly 170 bodies so far recovered. Nearly 1500 people including at least 700 foreigners were missing as of Thursday evening (NZ time).

Before-and-after satellite imagery of the region from Planet Labs, reviewed by Reuters, showed lush green mountainside now buried under brown debris and sediment.

"It’s confirmed that a substantial portion of that glacier snout collapsed to initiate the event - probably including or entraining a fair amount of rock and sediment," said Vikram Gupta, who specialises in engineering geology and teaches at India's Sikkim University.

Earth scientist Dan Shugar, an associate professor at the University of Calgary, said the imagery seemed to show that a large amount of snow had melted in the 24 hours prior to the disaster.

"Some of the glaciers in the valley were covered in snow yesterday and today they are mostly bare ice. So in other words (and I haven't seen any weather station data yet), it was probably warm," he said.

Nepal's snow-capped mountains had lost close to one-third of their ice in just over 30 years due to global warming, the United Nations said in 2023. The agency added that year that glaciers in Nepal, wedged between two major carbon emitters - India and China, melted 65% faster in the last decade than in the previous one.

"What I can see in this morning's satellite imagery from Planet Labs, the lower part of a glacier broke off at about 5200 metres and crashed onto the valley floor about 1200 metres below," Shugar said.

He said construction could be a factor too and "whether climate change played a role is something we will no doubt be looking into in the near future".

'ICE-ROCK AVALANCHE'

Andrew Zolli, Planet's chief impact officer, said his organisation would be releasing fuller imagery soon under an open licence to aid frontline response.

"Dan's analysis suggests this may have been the result of a glacier collapse, due to rapid melting," Zolli wrote on LinkedIn.

"Whether that, in turn, had a climate component is something that I'm sure will get scrutiny in the days to come."

Several hydropower projects are located in the worst-affected areas in the mountainous district of Rasuwa, which has a population of about 50,000.

Nepal's National Disaster Risk Reduction and Management Authority said the "glacier-inclusive flood" occurred in the Lhende River due to a landslide that took place about 20km northeast of a Nepal-China border crossing.

"The flash floods were caused by an ice-rock avalanche on the Nepali side," said Prakash Pokharel, a geologist at the authority who has reviewed the Planet Labs images.

Floods, landslides, avalanches and droughts are increasing in frequency and ferocity across the Hindu Kush Himalayas, and accelerating changes to water, land and air across the region pose a major threat to people, the International Centre for Integrated Mountain Development says.

The group said on Wednesday the flash flood sent a powerful surge of water, sediment and boulders through the Bhote Koshi and Trishuli river systems, adding that water levels on the river at the Galchhi area downstream reportedly rose by as much as 9 metres within 30 minutes.