A school ski trip turned tragic Tuesday as a bus carrying kids home to Belgium crashed in Switzerland, killing 28 people.

Among the dead are 22 children, most around 12 years of age. Ten of the children are Dutch, according to the Dutch foreign ministry. The two bus drivers were also killed.

The bus smashed into a tunnel wall in the Valais region of Switzerland at 9 PM local time, trapping many of the 52 passengers inside. Some 200 police, firefighters, doctors and medics worked through the night, while ambulances and helicopters took the injured to hospital.

"It is a sad day for all of Belgium," Belgian Prime Minister Elio Di Rupo said in a statement.

European Commission president Jose Manuel Barroso voiced sent his "sincere condolences and deepest sympathy" to the victims' relatives, and the European Parliament observed a minute's silence at noon.

The bus was one of three returning from the trip. The other two arrived safely. The operating company, Toptours, had "an excellent reputation," said Belgian Transport Minister Melchior Wathelet.

"The drivers had arrived the night before and had rested during the day before departure," he added. "It seems that the law on driving and rest periods has been respected." Witnesses report light traffic in the tunnel at the time of the collision.

It was the worst single crash in Switzerland since 1982, when 39 German tourists were killed as their bus was struck by a train at a railway crossing.

- With files from Retuers