Twenty-eight people, including 22 children, died in a coach crash in the south of Switzerland as they were returning to Belgium from a skiing holiday last night, Swiss police said.

Another 24 children were reported injured in the crash.

The bus, which was carrying 52 passengers, was travelling from Val d'Anniviers towards the Swiss town of Sion on the A9 motorway when the accident happened at 9:15 pm.

The bus suddenly swerved to the right and smashed into the wall of the tunnel. The impact of the crash was so violent that the front of the bus was seriously damaged, trapping many of the passengers, said a police statement.

"This is a tragic day for all of Belgium," said a statement from Belgian Prime Minister Elio Di Rupo, adding that he would be travelling to Switzerland today.

Police and fire services turned out in force to seal off the scene of the accident, closing the tunnel at both ends, as eight helicopters and 12 ambulances ferried the injured to four hospitals, police said.

Fire crews had to cut free some of the injured from the wreckage.

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. The students came from two different schools.

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

Psychological counsellors were on hand to help families of the children as they arrived in Switzerland.

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

Belgium's ambassador in Switzerland Jan Luykx went straight to the site of the accident.

"This tragedy will hit the whole of Belgium," he told the Swiss news agency SDA-ATS. "The magnitude of the accident is difficult to digest ... for the moment I am concentrating on the practical aspects.

"The emotional side will come when we meet with the families," he added.

Investigators do not yet know why the bus crashed.

The coach, which was registered in Belgium, was carrying 52 passengers from the cities of Lommel in northeast Belgium and Heverlee in the centre of the country.

Officials said rescuers worked throught the night.

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

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