The death toll from last week’s catastrophic flooding in Nepal has risen to at least 903, while the number of people reported missing has nearly doubled to 4,247, officials said Monday.

Nepal’s National Disaster Risk Reduction and Management Authority said 592 foreign nationals were among those still unaccounted for. About 85 Americans also remained missing as of Sunday, according to U.S. State Department spokesperson Tommy Pigott.

Search-and-rescue teams continue to face major challenges reaching isolated communities several days after the disaster struck the Nepal-Tibet border region on Wednesday.

Authorities say the massive flooding was triggered by a glacier collapse, unleashing a destructive surge of water, mud and debris that swept through communities downstream.

Ferrin Cole, an operations director with U.S.-based Crisis Response International, described the scale of the destruction as overwhelming, saying entire communities had been devastated by floodwaters rushing down from the mountains.

Emergency teams are working to reach survivors and deliver food, medical supplies and other essential assistance to affected areas.

The Nepal Red Cross estimates that more than 90,000 people have been affected by the disaster.

The mounting death toll has also placed severe pressure on morgues in some parts of the country. A police officer in Chitwan district told Reuters that more than half of the bodies recovered there were beyond recognition.

Across the border, Chinese authorities have separately reported at least 16 deaths and 546 people missing in Tibet, including hundreds of foreign nationals.

It remains unclear whether some people are included in both Nepalese and Chinese missing-person figures.

With thousands still unaccounted for and rescue crews struggling to reach remote areas, authorities fear the casualty figures could rise further as recovery operations continue.