Twenty-eight people, including 22 children, returning from a skiing holiday died in a bus accident in Sierre in the Swiss canton of Valais, the Swiss news agency SDA-ATS reported on Wednesday.
The children were travelling from Val d'Anniviers and were returning to Belgium.
The bus carrying the passengers was registered in Belgium, the news agency said.
The bus was travelling towards Sion, and hit a wall on Tuesday night in the motorway tunnel running from east to west and exiting at Sierre.
Valais cantonal police initially reported only injuries.
The bus was carrying 52 passengers from the cities of Lommel and Heverlee in Flanders.
The news agency said two drivers were killed in the accident.
Four Valais hospitals were treating the injured, SDA-ATS reported.