A massive flood and landslide disaster unleashed on the Nepal-Tibet border killed nearly 100 on Wednesday and left several hundred people missing, many of whom are foreign tourists.

Surveillance footage verified by AFP shows dozens of people running away as a stories-tall surge of debris slams into the Chinese side of a border area, tossing lorries and buses aside as it strikes.

Chinese state media reported "major casualties" from a mudslide at a Tibet border hub, reporting three dead and another 265 people missing.

Nepal's Prime Minister Balendra Shah vowed a maximum response, writing on social media: "The pain and loss you have suffered is not small. But I want to assure you that you are not alone in this difficult time -- the state is with you with all its means and resources." 

Police in Nepal reported the death toll hit at least 95, with rescuers wading through mud and sludge to look for survivors in a flood that buried lower floors of multi-story buildings.

Some 403 tourists -- 341 of them foreigners -- were unaccounted for several hours after the disaster struck, according to the ministry of culture tourism and civil aviation.

Nepal's tourism board said Americans, Indians, Malaysians, Britons and an Australian were among those unaccounted for, as well as 62 Nepali travellers.

Locals were left traumatised by the calamity, while experts cautioned that a lingering blockage in a river could produce an additional disastrous flood. 

"I got a call from (my brother-in-law) this morning, and they were crying and said their entire house had been swept away. Three family members were rescued, but his brother is still missing," said 56-year-old local Raju Bhadel.

The cause of the flooding was not immediately clear but Nepal's foreign minister Shishir Khanal said an earthquake likely triggered a landslide, which in turn blocked a river, sending water gushing downstream. 

The National Disaster Risk Reduction and Management Authority said satellite data from Planet Labs indicated a "snow-rock landslide" on the Nepal-China border may have caused the flood.

Images shared by Nepali police showed floodwater surging through a river, sweeping away houses in its path, while AFP images depicted the top floor of a house sticking out from muddy soil.

The Nepali Army was carrying out flights and had rescued 114 people from one of the worst-affected Rasuwa district, according to the prime minister's secretariat.

A police spokesman told AFP earlier that authorities had "alerted people who are staying near the riverside to relocate".

The Nepal Electricity Authority said about 430 megawatts of production capacity, including from hydropower and solar facilities, has been damaged.

"The disaster has damaged the production, transmission and distribution facilities of the Authority... resulting in disruption of electricity supply," it said.

Sarthak Shrestha, a remote sensing and geo information analyst at the Kathmandu-based International Centre for Integrated Mountain Development (ICIMOD), said a preliminary assessment showed "no anomalies" from China's side of the border. 

"We believe a rock-ice avalanche occurred in Nepal, blocking the Lhende river and causing the cascading flood event," he told AFP. 

Mudslide risk

Several highways were also closed, Nepali authorities said.

Chinese state broadcaster CCTV reported on Wednesday that "a mudslide disaster on the Nepalese side resulted in major casualties and missing people at the Gyirong Port" in Tibet.

Footage shared by the broadcaster, which AFP was unable to immediately verify, showed rushing water carrying tree branches and debris, as well as a flooded road.

China's leader said that "the most urgent task is doing everything possible to search for and rescue the missing people... quickly treat injured people and minimise casualties to the greatest extent possible", according to CCTV.

Traffic authorities in Tibet's Shigatse said roads in the area were "at risk of mudslides and unsafe for passage" in a post to social media on Wednesday.

In July last year, 17 people went missing after a mudslide near the Gyirong Port, Xinhua reported at the time.

Monsoon rains from June to September regularly trigger deadly floods and landslides across South Asia.

Scientists say climate change is increasing the frequency and intensity of extreme weather events in the region.

Qianggong Zhang, head of climate and environmental risks at ICIMOD, said a second flood may be imminent.

"With a blockage still lodged upstream on the Nepal-China border river, authorities warn a second flood may occur," he told AFP.