A bus carrying Belgian children home from a school ski trip crashed into the wall of a tunnel in the Valais region of Switzerland, killing 28 people, 22 of them children, police said on Wednesday.

The bus, transporting 52 people, mostly school children aged about 12 from Lommel and Heverlee in Flanders, crashed late on Tuesday evening in the canton of Valais, which borders Italy, police told an early-morning news conference.

It is an event which will really shake the whole Belgian population when they wake up this morning to such extremely sad news, Belgian Foreign Minister Didier Reynders told French radio Europe 1.

Reynders said the bus was one of three travelling together and the other two had returned to Belgium.

The bus was heading back to Belgium from a skiing holiday camp in Val d'Anniviers, a Valais ski resort. The front third of the bus was completely smashed in.

The crash was one of the worst since 1982 when 39 German tourists were killed on a railway crossing when a train hit their a bus.

Twenty-four more children aboard the Belgium-bound bus were injured, some seriously, and were being treated in hospital, police said. The bus was travelling on a highway towards Sitten from Siders when it crashed into the tunnel wall.

Two drivers in the bus also were killed. The cause of the accident was not yet known, police said.

Some 200 rescuers worked through the night at the crash scene, while 12 ambulances and eight helicopters ferried the injured to hospital.

The Belgian ministry of defence said it would make available two aircraft so that the families of the victims could be flown to the crash site later on Wednesday.

Switzerland's mountain regions have a history of deadly crashes. In 2005, a bus crashed in a Valais ravine, killing 12 and injuring 15. In 2001, a truck crashed in the Gotthard tunnel under the Alps, causing a blaze which killed 11 people.

(Reporting by Denis Balibouse, additional reporting by Ben Deighton in Brussels and Geert de Clercq in Paris, Editing by Michael Roddy)