The death toll from the flash flood that devastated Nepal and the Tibet region has risen past 470, as rescue efforts continue on both sides of the Himalayan border.

Nepal’s Prime Minister’s Office reported on Friday that 469 people had been confirmed killed and that 977 remain missing. At least three people were killed in Tibet and 558 remain missing, according to Chinese authorities.

In Nepal, 644 of the missing are foreigners, the authorities report. Around 1,550 people have been rescued.

The disaster struck on Wednesday, when a glacier collapse sent a surge of ice, rock and mud down Himalayan river valleys, burying entire villages and destroying roads, bridges and hydropower plants.

Bishal Nath Upreti, president of the Nepal Centre for Disaster Management, told Al Jazeera that the situation was “extremely difficult” for those stranded in the disaster zone, with no road to access them.

“Almost 42km [26 miles} of road is completely wiped out, and, except the helicopters, the rescuers cannot go there”, he said.

About 800 people have been rescued by helicopter and some 700 by land, he said. Aid to survivors is being airlifted to them.

“They are very desperate now,” he said of those stranded. “It’s already over 72 hours now and people are really desperate for … shelter and food.”

Al Jazeera’s Katrina Yu, reporting from Beijing, said the number of missing was expected to climb. Nepal has deployed 30,000 security personnel to search remote valleys.

China has deployed about 500 military and paramilitary personnel and sent Premier Li Qiang to supervise operations, she reports.

“That is very, very rare and it really gives us a sense of how urgent and how much of a priority this operation has become.”

‘Anxious ticking clock’

Rescue efforts are now being complicated by fears that secondary flooding could impact the disaster zone.

Attention is turning to a newly-formed barrier lake straddling the Nepal-China border. Both countries have warned it is unstable and at risk of bursting.

Barrier lakes form when flooding or landslides block a river, trapping a growing body of water behind a natural dam of debris.

Chinese authorities warned that the lake held around 2 million cubic meters (70 million cubic feet) of water, and was expected to receive another 3 million cubic meters (106 million cubic feet) over the next three days.

Meanwhile, rescue and military teams trying trying to reach the border port of Gyirong, where buildings lie buried under mud 1.5 metres deep, have been ordered to pause their work over fears of a mud slide, Yu reports.

“What this also introduces is a sort of anxious ticking clock,” Al Jazeera’s Yu reports, with teams racing to find survivors and recover bodies before more mud makes the operation more dangerous.

Waiting for answers

Nepal’s Foreign Ministry said on Friday that the country did not require international assistance with the search.

“The offer for help is natural. Our security agencies are doing the search and rescue operation. For now we don’t require such help,” spokesperson Lok Bahadur Chhetri said.

Al Jazeera’s Minelle Fernandez, reporting from Kathmandu, said Nepal was not entirely spurning offers of help, but had decided on the advice of its military and police that it could handle the initial rescue work itself.

“They have appealed for other forms of aid, for maybe cash disbursements, because obviously this is a mammoth task,” she said.

Standing near Tribhuvan University Teaching Hospital, she said crowds had gathered around the facility to inquire about missing loved ones.

“Everyone is fighting for answers, waiting for some glimmer of hope, and right now that’s hard to come by,” Fernandez said.