A bus carrying Belgian tourists 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 from Lommel and Heverlee in Flanders, crashed on Tuesday night in the canton of Valais, which borders Italy, police said.

The bus was heading back to Belgium from a skiing holiday camp in Val d'Anniviers, a Valais ski resort.

24 more children onboard the bus were injured, some seriously, and were being treated in hospital, police said.

The bus was traveling towards Sitten from Siders when it crashed into the tunnel wall.

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