On Wednesday, part of a glacier collapsed high in the Himalayas, triggering catastrophic flooding along the Nepal-Tibet border.

At least 160 people have been confirmed dead, while nearly 1,500 people are missing across Nepal and China’s Tibet, including at least 800 foreign nationals.

In Nepal, 826 people are missing, including almost 600 foreigners, according to police and tourism officials. Across the border, Chinese state media reported that 558 people were missing in Tibet’s Gyirong County, including 260 foreign nationals.

Here is what we know about what triggered the disaster, where the floods struck, and the risks facing the rapidly warming Himalayan region.

What happened in Nepal?

At about 8:40am local time (02:55 GMT) on Wednesday, flash floods and mudslides swept through mountainous areas along the Nepal-Tibet border, killing at least 157 people in Nepal and three in Tibet and leaving nearly 1,500 people missing across the region.

A massive flood of water, mud and rocks swept through the Lhende Khola river valley, engulfing villages and destroying homes and critical infrastructure. At least 19 bridges and about 40km (25 miles) of roads were washed away.

Nepalese soldiers and rescue teams have been deployed with helicopters, drones and search dogs, but high river levels, damaged roads and mud have complicated efforts to reach some of the worst-hit areas.

Across the border in Tibet, deep sediment has also delayed Chinese rescue teams trying to reach the border zone.

What triggered the mudslide and flash floods?

Officials were caught off-guard by the floods on Wednesday because it was not raining, according to local news editors.

The disaster appears to have begun high in the Himalayas, where part of a glacier collapsed, triggering a chain of events that caused a powerful surge of water and debris downstream.

Satellite imagery showed that a substantial section of the lower end of the glacier broke away at an elevation of about 5,200 metres (17,000 feet), plunging about 1,200 metres (3,900 ft) onto the valley floor, gathering rock and sediment as it fell.

The resulting ice-rock avalanche struck the Lhende River in Tibet, about 20km (12 miles) northeast of the Nepal-China border crossing. Debris temporarily blocked the river before the natural dam gave way, releasing a powerful flood downstream.

Early reports suggested that a magnitude 4.4 earthquake had triggered the collapse. However, the United States Geological Survey (USGS) later revised its assessment, saying that the seismic signal had been generated by the landslide itself rather than an earthquake.

The landslide eventually registered as a magnitude 5.2 seismic event.

Where exactly did the floods happen?

The disaster struck the mountainous Nepal-Tibet border region, with some of the worst damage in Nepal’s Rasuwa district, immediately south of the Chinese border.

The initial surge entered the Lhende Khola, which feeds into the Bhote Koshi and then the Trishuli River, carrying floodwaters south through Nepal. Water levels in the Trishuli reportedly rose by as much as 9 metres (27 ft) in just 30 minutes.

In the Rasuwa district, Timure and Syapru Besi were among the affected settlements. Further downstream, flooding reached parts of Nepal’s Nuwakot and Dhading districts, damaging homes, roads and other infrastructure.

Across the border in Tibet, mud and debris hit Gyirong Port in the Shigatse region.

The Trishuli eventually joins the Narayani River, which crosses into India’s Bihar state, where it is known as the Gandak River.

What do we know about the victims?

At least 160 people have been confirmed dead, while nearly 1,500 remain missing on both sides of the Nepal-Tibet border.

• Nepal: Police have recovered at least 157 bodies.
• Tibet: Authorities have reported at least three deaths in Gyirong County, where 558 people are missing, including 260 foreign nationals.

“My brother-in-law had a liquor store here. I got a call from him this morning, and they were crying and said their entire house had been swept away,” Raju Bhadel, a local businessman in Nepal, told Al Jazeera.

Manoj Mishra, a teacher, described seeing vehicles and people being carried away by the floodwaters in Nepal. “I saw around nine trucks being swept away by the floods, including people. It’s a heartbreaking incident,” he said.

Who is missing?

Nearly 1,500 people are missing across Nepal and Tibet, including at least 800 foreign nationals.

In Nepal, 826 people were reported missing, including almost 600 foreigners, according to police and tourism officials. The missing tourists come from more than two dozen countries.

Among those reported missing in Nepal are:

• India: 177
• United States: 63
• Australia: 34
• United Kingdom: 33
• Canada: 25

Authorities have not yet provided a complete nationality breakdown for all the missing foreign nationals.

Across the border, Chinese state broadcaster CCTV said 558 people were missing in Tibet’s Gyirong County, including 260 foreign nationals.

Many of those caught up in the disaster were pilgrims travelling towards Kailash Mansarovar in Tibet. Tourism officials are working to account for groups that had been travelling through the region when the floods struck.

The injured and rescued

At least 65 people were being treated for injuries in local hospitals and in Kathmandu.

More than 5,000 Nepalese army and police personnel have been mobilised for the rescue operation, while helicopters are searching for survivors and delivering emergency supplies to communities cut off by damaged roads and flooding.

Survivors from badly affected areas have also been evacuated by military helicopter, including some taken to hospitals in Kathmandu.

How are international organisations and leaders responding to the crisis?

Countries and international organisations have offered emergency assistance, funding and technical support as rescue operations continue in Nepal and Tibet.

Humanitarian and financial aid

India has sent a rescue aircraft carrying about 10 tonnes of emergency supplies to Nepal, including temporary shelters, blankets, hygiene kits, solar lamps and medicines.

Indian Prime Minister Narendra Modi spoke with Nepalese Prime Minister Balendra Shah and pledged India’s support.

The International Federation of Red Cross and Red Crescent Societies (IFRC) has released 1 million Swiss francs ($1.2m) in emergency funding to support the Nepal Red Cross Society’s relief operations and damage assessments.

International coordination and satellite support

The United Nations has deployed personnel in Nepal to support government-led relief efforts, while its office in China has said it is ready to assist rescue operations in Tibet.

UN Secretary-General Antonio Guterres expressed his condolences to those affected and said the organisation stood in solidarity with communities hit by the disaster.

The European Union has activated its Copernicus Emergency Management Service, which uses satellite imagery to map disaster zones and provide information that can help authorities assess damage and coordinate rescue operations.

Search-and-rescue efforts

South Korea has established a government task force after several South Korean nationals working at a hydropower project were reported missing. Seoul said it planned to send a rapid-response team involving government, fire and police personnel to Nepal to assist with the search.

The United States said it was sending a disaster response adviser and providing $500,000 in assistance.

Canadian Prime Minister Mark Carney said Canada was closely following the situation, describing the floods as “absolutely devastating”.

On the Chinese side, President Xi Jinping ordered an “all-out” search-and-rescue operation for people missing in Tibet and called for efforts to prevent further disasters.

What role did climate change play in this disaster?

Scientists are still investigating what triggered the glacier collapse, and it is too early to determine how much climate change contributed to this specific event.

However, the disaster comes against a backdrop of rapid warming and glacial loss across the Himalayas. Glaciers across the Hindu Kush Himalaya mountain range – which spans Afghanistan, Bangladesh, Bhutan, China, India, Myanmar, Nepal and Pakistan – lost ice 65 percent faster between 2011 and 2020 than during the previous decade. Nepal’s glaciers have lost close to one-third of their ice volume in roughly three decades.

As glaciers retreat, mountain slopes can become increasingly unstable, and meltwater can accumulate in or around glaciers, increasing the potential for sudden floods.

Satellite observations showed unusually warm conditions around the affected glacier before the collapse, with snow cover diminishing considerably and exposing more bare ice.

Researchers are investigating whether melting contributed to the glacier’s instability, but a direct link between climate change and this week’s collapse has not been established.

What is the situation now?

More than 5,000 Nepalese army and police personnel are searching for survivors and recovering bodies, while helicopters are delivering tents, food and other emergency supplies to communities cut off by the floods.

Officials have warned that the death toll is likely to rise as rescuers reach areas that remain inaccessible because of damaged roads, high water levels and thick layers of mud.

In Tibet, Chinese authorities are also monitoring a still-dammed lake upstream from the disaster zone in Gyirong that could complicate rescue efforts and pose a further flood risk.