At least 270 people have been killed and hundreds remain missing after catastrophic flooding tore through towns and mountain valleys along the border between Nepal and China’s Tibet region, officials said.

The disaster has triggered a massive search-and-rescue operation across the rugged Himalayan region, with authorities warning that the death toll could rise significantly as teams reach isolated communities.

More than 500 foreign nationals from 32 countries are among those reported missing in the region, a popular destination for pilgrims and trekkers. According to the Nepal Tourism Board, the missing include 65 Americans, while 127 Nepalese travelers are also unaccounted for.

Chinese authorities reported three deaths and said two people had been rescued following a major mudslide at a border crossing in Tibet’s Gyirong County. Officials said 558 people remained missing there, including 260 foreign nationals.

It was not immediately clear whether the casualty and missing-person figures released by Nepal and China overlap.

Videos from Nepal’s hardest-hit Rasuwa region showed the scale of the destruction, with raging floodwaters carrying mud and debris through communities and tearing apart bridges.

The exact cause of the disaster remains under investigation. Nepalese authorities said preliminary indications suggest the flooding may have been triggered by a landslide associated with a glacial event in the Himalayas.

Officials believe the event unleashed a sudden and powerful surge of water downstream, devastating settlements and infrastructure along its path.

Emergency teams are continuing efforts to locate survivors and determine the full extent of the destruction as concerns grow for hundreds of people still unaccounted for.