Geneva: Twenty-eight people, including 22 children, returning to Belgium from a skiing holiday died in a bus accident in Sierre in the Swiss canton of Valais, Swiss police said on Wednesday.

Another 24 children were reported injured. They were travelling from Val d'Anniviers.

During a press conference early this morning, the Valais police commander said the tragedy was "unprecedented" and that even seasoned rescuers had been traumatized.

Helicopters and ambulances ferried the injured to four hospitals, police said.

In Brussels, the Belgian foreign ministry said most of the children were aged around 12 and the bus was one of three hired by a Christian group.

Surgeon Jean-Pierre Deslarzes said in one of the hospitals: "All the rescuers were shocked by what they have experienced."

"This tragedy will hit the whole of Belgium," the Belgian ambassador to Switzerland Jan Luykx told the Swiss news agency SDA-ATS. "The magnitude of the accident is difficult to digest."

The bus, registered in Belgium, was travelling towards Sion and hit a wall last night in the motorway tunnel running from east to west and exiting at Sierre.

The bus was carrying 52 passengers from the cities of Lommel in northeast Belgium and Heverlee in the centre of the country.

Officials said rescuers worked through the night.

The news agency said two drivers had been killed in the accident.