Massive flash floods in a border area of Nepal and China on Wednesday killed at least 160 people and left hundreds of tourists, workers and others missing after sweeping through communities and bringing travel in some areas to a halt. Experts said an avalanche from a glacier was the likely cause in the warming Himalaya region.

Police in Nepal reported at least 157 dead. China’s state broadcaster CCTV reported at least three dead and 265 missing in the Tibet region’s Gyirong County near the border. Drone footage showed roads destroyed and buildings buried by thundering water the color of wet cement. State news agency Xinhua reported significant casualties.

The region’s steep valleys are economic lifelines and travel corridors but precarious ones when disaster strikes. Images showed what appeared to be the second floors of buildings in Nepal’s hard-hit Nuwakot district slathered in mud after the torrent rushed by. Debris hung from tree branches and wires, vehicles were buried and some stunned survivors were caked in mud.

There was little time for anyone to flee.

“When I reached Dhunge bazaar, in half an hour, the flood came and damaged all the market. People were preparing to go to office then,” resident Shoyam Rajbhandari said. “How many people are missing or dead, we don’t know.”

Many missing were on pilgrimage to a sacred site

Officials in Nepal said 403 people, including 341 foreign nationals, were reported missing. Sunil Sharma, a tourism board spokesperson, said 133 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.

The State Department said it was aware of Americans affected.