A bus crash in Switzerland has killed at least 28 Belgian tourists, 22 of them children, according to reports.

The bus, carrying 52 people back from a ski camp, crashed head-on into a wall of a tunnel near Sierre on Tuesday night at around 9 p.m., the Associated Press reported. The area is in the southern Swiss canton (state) of Valais, a popular destination for skiiers.

In Brussels, the Belgian foreign ministry said most of the children aboard were aged around 12. Ten of them are Dutch, according to the Netherlands' foreign ministry, which added that all but one of the Dutch children lived in Belgium.

The bus was one of three hired by a Christian group, the Agence France-Presse reported, citing the Swiss news agency SDA-ATS. It was reportedly en route from Val d'Anniviers back to the Belgian provinces of Brabant, Flanders and Limbourg.

Another 24 children were hurt in the crash. Some of the injured were flown by helicopters to hospitals in Lausanne, Berne and other Swiss cities.

More than 200 rescuers were reportedly working at the scene of the accident.

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The AP reported that the highway was closed in both directions, with up to eight helicopters and a dozen ambulances brought in to ferry victims to hospitals. Dozens of firefighters and police, 15 doctors and three psychologists were also called to the scene.

Didier Reynders, the Belgian foreign minister, was quoted as saying: "It is incomprehensible. There were three buses and only one was in [an] accident without any contact with another vehicle."

An investigation into what caused the crash is underway, the BBC said.

The vehicle collided with the right-hand wall of the tunnel before hitting a concrete wall that forms part of an emergency access section, Swiss prosecutor Olivier Elsig told a press conference. Driving conditions were reported to be normal at the time.

The coach's two drivers had taken rest periods as required by law, according to Belgian transport minister Melchior Wathelet. Both drivers were killed.

Belgian Prime Minister Elio Di Rupo called it "an extremely sad day for all of Belgium." He was reportedly flying together with the families of the victims to Switzerland on Wednesday.