The unprecedented deluge of ice, rock, mud and debris on the Nepal-Tibet border on Wednesday - blamed on a glacier collapse - killed more than 900 people, Nepal's disaster agency said on Monday.
More than 4700 remain missing on both sides of the border.
Hundreds of bodies, many unidentified, have been buried in shallow graves as they began decomposing in the humid weather in the plains of the Himalayan country after being swept down the mountains by the torrent.
Nepali officials said the focus was on reaching the hundreds of workers believed to be trapped inside about half a dozen hydropower project tunnels that had been blocked by mud and rock debris.
Pictures and videos shared by rescue teams showed earth-moving equipment digging and shovelling mud and rocks under lights in the dark of the night as workers watched. Soldiers used torchlight in a big pit they had created to look for space to get into one tunnel.
Nepal's disaster management authority said 933 people were missing from hydropower projects in the flood-hit region.
The Red Cross has estimated that more than 90,000 people are likely to have been affected by the disaster, which China has linked to the effects of climate change.
"A large amount of debris has been deposited and that is posing us a big challenge to open the tunnels," Nepal army spokesperson Raja Ram Basnet told Reuters.
"Our main focus is on opening the tunnels."
Prime Minister Balendra Shah said the flood was an "unprecedented and devastating natural disaster" and it remained challenging to compile a complete assessment of the damage.
"Despite the adverse weather, difficult terrain and continuing risks, we are moving forward with determination to save the lives of our citizens," he said in a Facebook post late on Sunday, adding that nearly 21,000 security personnel were involved in the rescue operations, besides civilian workers and volunteers.
Nepal's disaster agency said 903 deaths had been counted there, with 4247 people, including 592 foreigners, missing - including 43 Australians.
As of Sunday, Chinese authorities reported 16 deaths and 546 missing, and Chinese state media said 261 of the missing were foreigners, including over a hundred Nepalis.
Beijing has said 261 foreign nationals from 23 countries, were unaccounted for in Tibet, near a key border crossing with Nepal.
China's state-run CCTV reported Beijing has mobilised more than 2100 rescue workers and allocated at least 220 million yuan ($A46 million) to support relief efforts and has been airdropping equipment and supplies to frontline rescuers, while its soldiers used life-detection equipment to look for the missing.
Although Nepal has said that it does not need foreign help in general rescue and search, it has leaned on giant neighbours India and China for their expertise in tunnel rescue.
Rescue operations have been suspended several times since Friday because of bad weather and concerns that a lake formed across the Nepal-China border by the disaster could trigger fresh flooding after it began overflowing into Nepal's rivers.
The weather was good on Monday and rescue work continued to pick up pace, said Narendra Pariyar, the district administrator of Rasuwa, one of the two hard-hit districts.
In the holy town of Devighat, on the banks of the Trishuli river that was flooded, residents walked around the shells of what had once been homes, trying to salvage their belongings from the debris.
The flood was "caused by glacier instability under the long-term effects of global warming", Chinese state broadcaster CCTV reported, saying it was a "new and prominent feature of cryosphere disasters" on the Tibetan plateau.
Two Nepali officials told Reuters that China had not shared much information on glacier risks and water levels following a meeting to strengthen co-operation earlier this year, and they feared that a lack of data sharing could hamper future disaster preparedness.
The mudslide from the Nepalese side took six to seven minutes to reach the Gyirong border crossing, CCTV said, citing the findings of Chinese scientists.
Dramatic surveillance footage shared worldwide showed torrents of water and debris engulfing buildings while people tried to flee.
