More than 300 tourists, including 33 Britons and 54 Americans, are missing after devastating flash floods wiped out buildings and bridges along the Nepal-Tibet border. Experts believe a colossal glacier snapped, sending a 2,000ft chunk smashing 'like a bomb' into the valley more than a kilometre below, and triggering the catastrophic floods.Entire buildings and roads were swept away when a 30-mile torrent stormed down two rivers in the Rasuwa district of Nepal, north of Kathmandu, leaving a path of near total destruction in its wake.The area is popular with tourists who flock to the Himalayas for hiking, and the valley is littered with small villages and infrastructure projects.Terrifying footage shows a tsunami wave of mud crashing through towns in Rasuwa, sweeping away everything in its path. At least 157 people have been confirmed dead in Nepal, while at least 403 are missing - 341 of whom are tourists. The missing include 133 people from India who were on a pilgrimage, 34 Australians, 24 Canadians and eight South Koreans - alongside the Britons and Americans.Prime Minister Andy Burnham said the Government would do 'everything we can' to support Britons caught up in the floods. Meanwhile, China reported three people had been killed in Gyirong, Tibet, with 265 people still missing.A clip showed the Gyirong port land crossing being annihilated by the flood. Dozens of people were seen running away in terror seconds before the water and mud crashed into the structure and wiped it out. CCTV footage shows people running away as wall of mud and water destroys buildings in a valley at the Nepal-China border in Gyirong One clip showed the Chinese-owned Gyirong port land crossing being annihilated by the flood Several people have been killed by the flash floods, with many more casualties expected to be reportedHundreds of people are missing following the deadly floods in Nepal, which swept away large pieces of infrastructure Houses lie partially submerged in muddy water along a riverside after flash floods at Trishuli Bazar in Nepal's Nuwakot district