Rescuers in Nepal and China on Saturday battled to locate thousands of people missing days after a deadly, tsunami-like wall of water and debris swept through the Himalayan region, engulfing entire villages.

Figures from both countries put the number of missing close to 3,000, with 682 bodies recovered -- and reports of some dead possibly swept downstream as far away as India.

In a breakthrough late Saturday, the Nepali army was able to insert a pipe into a tunnel at the Trishuli 3A hydroelectric site where workers are believed to be trapped.

"We have already started sending air through the small opening, and we will now begin excavation work and send the rescue team inside," Home Minister Sudan Gurung said in a statement.

More than 100 workers may still be alive and trapped inside such tunnels, with rescue efforts focused on the Trishuli 3A and 3B sites.

The US Geological Survey said a glacial collapse was behind the deadly torrent on Wednesday at the Nepal-Tibet border.

While the exact cause of the collapse is unclear, climate change is causing glaciers to melt worldwide, increasing the risk of such disasters.

"The sudden glacial collapse and flash flood is a devastating reminder of how fragile these mountain environments can be", UN climate chief Simon Steill said in a statement.

"We also know a warming climate, driven by humanity burning colossal amounts of coal, oil and gas, is making tragedies like this one, and so many others around the world, much more likely, frequent and severe, with vulnerable communities often hardest hit," he added.

'Nothing left'
Survivors face an agonising wait to learn the fate of missing relatives, and the prospect of rebuilding their lives from nothing.

"All the settlements near the river were swept away," Buddhi Ram Dangol told AFP in Nepal's Nuwakot district. "There is nothing left."

At an army base serving as a key rescue hub in Bidur, relatives crowded around lists of rescued people pasted on a wall.

"I have come here looking for my son," said Kalka Sharda, scanning the list in desperation. "He was a labourer at the hydropower plant, but I can't find his name, and the authorities are not saying anything."

In an area home to vast energy infrastructure projects, Nepal's disaster authority listed 933 hydropower workers among the missing, along with 517 foreigners, including trekkers and Hindu pilgrims to Tibet's sacred Mount Kailash.

The Isha Foundation, led by Indian guru Sadhguru, said in an Instagram post on Saturday that 80 members had been on a group trip in the area.

The group was at the hard-hit Chinese border port of Gyirong, which has been "completely washed away", the post said.

"At this time, there seems to be no encouraging indications of survivors at the immigration office area."

'Enormous' trauma
The United States will provide $3.6 million in humanitarian aid to Nepal, the State Department announced Saturday, including emergency food assistance. This came after Washington pledged $500,000 in the initial aftermath of the disaster.

Nepal's Foreign Minister Shisir Khanal said the reconstruction bill could run into "billions of dollars".

"The urgency is on rescue and saving lives as much as we can," he said, appealing for specialised assistance, including heavy-lift drones and technology to help locate people who are trapped.

He also called for forensic testing kits to identify bodies and freezer units to store them, saying bodies that have been in water could decompose at a faster rate.

Fears of further flooding have compounded the search for the missing.

The deluge of ice and mud caused huge lakes, and one of these formations has become a risk of further floods.

The Chinese and Nepalese foreign ministers spoke Saturday and discussed sharing satellite and hydrological data, both sides said.

In Tibet, where Chinese authorities are the only source of information, rescue operations were temporarily paused at the hardest-hit area of Gyirong over the risk of more flooding from a newly formed lake.

But state broadcaster CCTV said late Saturday that the lake was "discharging naturally" and had been "essentially drained", citing China's ministry of water resources.

A total of 2,141 rescuers were near the Gyirong site, the Xinhua state news agency reported, including the more than 500 in the worst-hit area.

Pilgrimage route
Nepal reported a death toll of 675 people on Saturday.

There are 2,426 people, including hundreds of foreigners, who are out of contact in the country.

Across the border in Tibet, which is largely inaccessible to foreign journalists, seven people died and 554 remain missing, Xinhua state news agency said on Saturday.

Authorities also said they had evacuated 555 stranded tourists and 499 Chinese residents from Tibet.

In India, officials said they had recovered seven bodies from rivers flowing from Nepal, believed to be flood victims.

Hope was fading for some still waiting desperately for news.

Kalka Sharda, in Bidur, was searching lists of rescued for his 18-year-old son Vinod Sharda, a hydropower plant labourer.

"I don't think there is any hope left," he said.

"Because, if he was alive, he would have found a way to get in touch."