Teams in Nepal have been racing to save more than 100 workers trapped in a tunnel near a hydroelectric dam construction site, as fears of further flooding impede rescue efforts.

Chinese state TV said on Friday (Australian time) that China has suspended rescue operations because a dammed lake upstream of the Nepalese border crossing had begun to overflow.

The news come as the death ‌toll ‌from the catastrophic flash ‌flood ‌⁠in Tibet rises to 469, with searchers continuing to look for more than 1400 missing people.

The office of Nepal’s Prime Minister, Balen Shah, posted footage of the tunnel rescue to Facebook.

“A team of the Nepalese Army has successfully rescued 350 labourers and locals trapped in a tunnel a while back, facing an extremely dangerous and challenging situation amid adverse weather and floods,” the PMO said in the post.

“The troops are actively working to safely evacuate more than 100 people trapped in the same tunnel.”

It was later announced that Nepalese authorities had also suspended rescue operations due to the overflowing lake upstream.

Among those still missing after the disaster are 39 Australians, with Foreign Minister Penny Wong warning that number could rise. She said the Department of Foreign Affairs and Trade was assisting with 43 consular cases along the mountainous Nepal-China border.

“We are urgently working to identify how many Australians have been in the region and to confirm the welfare of any Australians who are not accounted for,” Wong told ABC Radio.

Nepal and China earlier warned of fresh risks of flooding as two lakes ‌that have developed on either side of the border threatened to burst.

“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,” Nepal’s disaster authority said.

Chinese officials have been monitoring rising water upstream from Gyriong port – the land border between Nepal and Tibet that was swept away in a torrent of mud and water on Wednesday.

China has 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.

The formation of the 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.

On Friday, rescuers were told to temporarily halt operations after hours of heavy rain, with fears the lake was about to overflow.

Elsewhere, rescue teams and military helicopters continued to search for hundreds of missing people after the deadly torrent of water and mud swept through communities.

The flood destroyed whole villages and towns, power stations, roads and bridges along a route that links Kathmandu with Lhasa in Tibet and is popular with trekkers and pilgrims.

Chinese state broadcaster CCTV said 558 people were missing in Tibet’s Gyirong county, including 260 foreigners. China said the death ​toll stayed at three.

In Nepal, 667 people are missing.

As well as the Australians, they include people from more than two dozen countries, including 288 from India, 63 from the US, 53 from Ukraine, 51 from Malaysia, 33 from Britain, 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.

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.

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 sai.d

Australia has established a $5 million assistance package for affected communities. It will provide humanitarian support including food, emergency shelter and medical support.

An initial team of three humanitarian response specialists will be sent to help 20 staff already on the ground in Nepal.

Sydney brothers Aarush and Ashwin Muralitharan were on a tour with their parents. Photo: Supplied

Sydney doctor Manoj Dharmaratnam, who was invited to join the 15 on the sacred pilgrimage but chose not to go, prescribed several of his close friends with altitude medication and conducted injury assessments before they left Australia.

“One of them had an injury and I did tell her, maybe you’re not fit to go on this trek,” Dharmaratnam said.

“She actually called up and cancelled her trip … but then she said ‘I’ve bought the ticket, so I might as well go’.”

Also missing are a respected Sydney couple with two sons in their 20s who were both studying medicine in NSW.

“They’re such pioneers in the community of ours. They’re very well known and well respected, so it’s such a shame,” Dharmaratnam said.

The last contact with the family came at 8.30 local time, when one of the sons informed his girlfriend they were checking out of their hotel in Nepal’s Langtang Valley.

“We just want to know something…that’s all we’re trying to do, you know, we just want to known they’re safe,” Dharmaratnam said.

“Miracles do happen and we’ve heard miracles are happening, so fingers crossed we are hoping they’ll be fine.”

The region was “extraordinarily remote and very difficult to traverse”, complicating rescue efforts, former Australian ambassador to Nepal Felicity Volk told ABC Radio National on Friday.

“The terrain is so difficult to traverse that it will make, as we’re seeing, this rescue effort almost impossible without roads, without communications,” Volk, who is director of the Australian Himalaya Foundation, said.

“That will also make it very difficult for the Australian government to access any information about the missing Australians.”

Nepalese-Australian families gathered for prayers and held a minute’s silence for those lost to the floods on Thursday night.

Keshav Kadel was among members of Nepalese community who gathered at the Australian Nepalese Multicultural Centre, which he serves as vice-president.

“What’s happening in my town is that bodies are now flowing in the river,” he said.

“They have slid all the way down and people are pulling bodies from the river.”

Smartraveller has updated its travel advice, urging people to exercise a high degree of caution because of major flash flooding and a mudslide.

Australians in need of urgent help should contact DFAT’s 24-hour consular emergency centre.

-with AAP