A 13-year-old girl is among 33 British people feared missing after flash floods tore through villages on the border between Nepal and the Tibet region.
At least 95 people have been killed and hundreds of people, including two Irish nationals are also unaccounted for in the crisis in Nepal’s Rasuwa region.
Dramatic video footage showed a wall of water crashing through river banks, bringing down bridges, homes and other infrastructure in villages along the Bhotekoshi River.
Climate experts say an avalanche from a glacier is likely to have caused the disaster.
Security footage captured on the Chinese side of the Nepal-Tibet border showed many people fleeing before a torrent of rock, mud and water destroyed buildings, vehicles and took lives.
With the deluge of muddy water sweeping down from an estimated 6,560ft above sea level, flood alerts were issued in the populous northern Indian states of Uttar Pradesh and Bihar, which border Nepal and where several rivers flow down the mountains into the plains.
Authorities in Bihar said they had got about 5,000 people from six districts out and planned to get out a total of 10,000 to 12,000 people.
Rescue teams in Nepal had recovered 95 bodies so far, police said. Local media reported 65 people were being treated in hospitals and in the capital Kathmandu.
Alpine Eco Trek, which runs guided tours and expeditions in Nepal, said initially that 12 British citizens on a trip across the Nepal-Tibet border were missing.
Nepal’s tourism board said 384 people were unaccounted for.
Ram Kumar Adhikari, manager at Alpine Eco Trek, said two Irish nationals alongside four South Africans on the trip were believed to be missing.
He said there was "not much info to share at the moment, but yes we have 12 British citizens (supposed) to make entry this morning".
Travel logs show the ages of the missing British citizens range between 13 and 65.
The firm, which also leads tour groups up Mount Everest, confirmed four of its Nepalese tour guides – aged 38, 35, 45, and 29 – are missing as well.
The Nepal Tourism Board said its staff and tourist police are “working to verify the whereabouts and safety status of tourists and travellers who were in or travelling through the affected areas”.
A preliminary check found 291 foreign nationals had been reported missing in the affected areas, with more than 100 still to have their nationality confirmed.
Sunil Sharma, a tourism board spokesperson, said 133 people were from India on pilgrimage to Mount Kailash in Tibet, a sacred site for Hindus. Also missing were 47 from the United States, 34 from Australia, 33 from Britain and 24 from Canada, he said.
Officials issued warnings to residents of several of Nepal’s districts and asked them to stay away from the banks of Bhotekoshi, Trishuli and Narayani rivers.
The Nepalese government said it had asked both India and China to help with the rescue efforts.
Indian Prime Minister Narendra Modi spoke with Nepal's Prime Minister Balendra Shah and offered help. "India is prepared to provide Nepal with all possible types of humanitarian assistance. Our teams are closely coordinating on rescue and relief efforts," Modi said on social media.
Eight South Koreans working on a hydropower plant were also missing, and 10 others were stranded, following the massive flash flood, according to South Korea’s Foreign Ministry.
The US Geological Survey had reported a magnitude-4.4 earthquake early on Wednesday, about 60 miles north of Kathmandu, near the border with China's autonomous region of Tibet.
In a joint statement with Australia, the EU, Finland, France, Norway and Switzerland, the UK embassy in Nepal said: “We are deeply saddened by the devastating flooding in Rasuwa and Nuwakot districts and surrounding areas.
“Our thoughts are with all those affected by this disaster.
“We express our solidarity with the people of Nepal during this difficult time.
“We encourage all those in affected areas to follow the guidance of local authorities and official safety advisories as the situation continues to evolve.”
Climate experts at the Kathmandu-based International Centre for Integrated Mountain Development said an avalanche from a glacier was likely to have caused the flash flood.
The Lhende Khola River, a tributary of the Bhotekoshi, had maximum flooding on Wednesday, they said.
"The Lhende Khola has flooded twice in 14 months, this time triggered by an ice rock avalanche from a glacier on the Nepal side that blocked the river and released a sudden surge downstream," said Saswata Sanyal, a disaster risk reduction specialist at the climate research group.
He noted the importance of early warnings in "a warming Himalaya".
The region suffered similar flash floods in August last year, when a glacial lake in Tibet overflowed because of high temperatures.
Nine people were killed, and critical infrastructure, including a bridge connecting Nepal and China, was damaged.
The Himalayan region, which spans several countries including India, is especially vulnerable to heavy rains, floods and landslides because of warming due to human-caused climate change.
Research shows that extreme weather events, including heatwaves, rains and melting glaciers, have become more frequent.
Researchers in Kathmandu found earlier this year that glaciers across the Hindu Kush Himalaya region are melting at an accelerating rate, with ice loss rates doubling since 2000, primarily due to rising global temperatures and climate change.
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