Rescue teams and military helicopters are continuing to search for hundreds of missing people along the mountainous Nepal-China border, where a deadly torrent of water and mud swept through communities a day earlier following a glacier collapse.
The flood ripped through villages and valleys, destroying power stations, roads and bridges along a route that links Kathmandu with Lhasa in Tibet and is popular with trekkers and pilgrims.
Nepal and China have warned of fresh risks of flooding in the Himalayas as two lakes that have developed on either side of their border threaten to burst in a region grappling with a catastrophic flood.
The tally of missing in Tibet's Gyirong county was 558, Chinese state broadcaster CCTV said on Thursday, including 260 foreigners.
China said the death toll stayed at three.
In Nepal, 667 people were missing.
The missing foreigners in Nepal belonged to more than two dozen countries, including 288 from India, 63 from the United States, 53 from Ukraine, 51 from Malaysia, 39 from Australia, 33 from the United Kingdom, 25 from Canada and five from New Zealand.
The Nepal Tourism Board said 31 foreigners who had previously been reported missing had been rescued.
They included two Italians and a Swiss citizen.
Nepali Prime Minister Balen Shah said in a post on Facebook that more than 13,000 army and police personnel have been mobilised for the rescue operations.
He said 573 people had been rescued on Thursday in 69 relief flights.
Nepal's disaster authority issued a notice that a lake formed by debris blocking a river at the flood site is rising and at risk of bursting.
"Residents of the lower banks ... passengers, rescuers engaged in rescue operations, and all concerned are requested to exercise special caution and stay in safe places away from the river banks and risky areas," the authority said.
The United Nations' humanitarian chief, Tom Fletcher, ordered the release of $US2 million from the UN's Central Emergency Response Fund to help with medical care, shelter and water.
He called the floods "horrifying."
"We pledge to support national efforts to deliver emergency aid and support," UN Secretary-General Antonio Guterres told reporters.
President Donald Trump said the US had offered support.
"Nobody's ever seen anything quite like that, where very powerful, strong, well-built buildings were wiped out like they were toothpicks. It's a terrible thing. We are sending aid and we've spoken to the leadership and we let them know anything they want, they'll have."
China warned three million cubic metres of water - the equivalent of about 1200 Olympic swimming pools - are expected to flow into an already dammed lake on its side of the border over the next three days, creating a high risk of a breach, with peak flow expected on Tuesday.
The formation of a barrier lake was captured in aerial footage by a Chinese rescue team on Thursday, state broadcaster CGTN reported.
The footage showed masses of brown water accumulating in a basin with no visible outlet.
The water had already submerged bushes and trees and appeared to push against a barrier of debris and stones.
Rainfall forecast over the coming week will exacerbate the threat, Chinese state media said, citing the Chinese water resources ministry.
"The volume of water is continuing to increase, and so is the risk," Zhu Jinfeng, a researcher at the water resources ministry's hydrology department, told state broadcaster CCTV.
with AP and DPA
12°C