A devastating bus crash in the Swiss Alps has left at least 22 Belgian schoolchildren dead. A further six people died in the crash on Tuesday night in a tunnel in the south-western canton of Valais, or Wallis. The classes of 11- and 12-year-olds were returning from a school skiing trip to the small Flemish towns of Heverlee and Lommel, east of Brussels. Distraught parents gathered at the Saint Lambert school in Heverlee anxiously awaiting news of who had died and who had survived the crash, which is believed to be the worst road accident in Switzerland\'s history. The bus was said to have been driving at high speed through the tunnel when it veered into the concrete tunnel wall. No other vehicles were believed to have been involved. \"A very sad day for all of Belgium. Words are not enough for this terrible accident,\" said the prime minister, Elio Di Rupo, before flying to the scene of the tragedy. Two military aircraft were made available to ferry parents of the victims to Switzerland. Didier Reynders, the foreign minister, said the bus was alone in the tunnel at the time of the crash. \"We have no indication of what caused the crash.\" There were 52 people on board the bus, overwhelmingly schoolchildren. Of the 28 who were killed, 22 were pupils. The other six dead included the two bus drivers. The remaining 24 were all injured, some very seriously. Many of them were taken by helicopter to Swiss hospitals. While most of the children were from two Flemish schools, there were also seven Dutch children on the bus. The crash took place as the children were returning from a skiing trip in Val D\'Anniviers. As the parents of the schoolchildren congregated at the two Belgian schools this morning, it remained unclear who had lost sons or daughters. \"The information at the moment is very scarce,\" said a police spokesman in Heverlee, just outside the university town of Leuven. There were 24 children from the Heverlee school on the bus and 22 from a Lommel primary school. \"Unbelievable,\" said the mayor of Lommel, Peter Vanvelthoven. \"We don\'t know yet how many children from this school are victims.\" Psychologists and medical personnel were on hand at both schools to support and assist the bereaved.