The death toll from catastrophic flash floods that struck Nepal on Wednesday has risen to 538, while 977 people remain missing, Nepal's National Disaster Risk Reduction and Management Authority said Friday. The authority said 14,829 security personnel have been mobilised for search and rescue operations, while 3,742 people have been rescued so far. The disaster, triggered by a massive collapse of bedrock beneath a glacier in the Himalayan region, unleashed torrents of rock, mud and water that swept away buildings, bridges and roads in parts of Nepal and neighbouring Tibet. Rescue operations continued Friday as helicopters evacuated stranded people and security forces searched for survivors in the hard-hit areas near the Nepal-China border. Nepali police also issued a fresh alert after receiving reports that a dam on the Tibet side had burst, raising fears of further flooding. Authorities had earlier warned residents about a barrier lake that formed after the initial disaster and had begun overflowing. Among those still missing are hundreds of foreign nationals who were in the region for trekking, work or pilgrimages, including trips to the sacred Mount Kailash in Tibet. Nepali Army personnel were also working to rescue people trapped inside a hydropower tunnel in Rasuwa, where thick mud and debris complicated efforts to reach survivors. Scientists analysing satellite images said the collapse beneath the glacier caused rocks and melted water to surge into rivers in the valleys below, creating a devastating torrent that swept through settlements in Nepal and Tibet.