A bus carrying a Belgian school party home from a ski trip crashed into the wall of a tunnel in Switzerland late on Tuesday, killing 28 people, mostly children.

Distraught parents, many of whom still did not know if their children were among the 22 pupils killed, gathered at a school in Belgium to be flown to Switzerland on military aircraft.

"Some parents know their kids have survived, but for others there is no news," said Belgian police spokesman Marc Vranckx.            

The bus, transporting 52 people, mostly children aged about 12 from the towns of Lommel and Heverlee in Belgium's Dutch-speaking Flanders region, crashed in the Swiss canton of Valais at 9:15 pm (2015 GMT).

A police photograph showed the bus rammed up against the side of a tunnel, the front ripped open, broken glass and debris strewn on the road and rescue workers climbing in through side windows.

Children at St Lambertus school in Heverlee, a suburb of Leuven, were informed about the accident at an assembly before classes. Flowers were laid outside the Catholic school where eight children were still unaccounted for.

"The eight sets of parents, they can only sit and wait, they just don't know. I'm in pain, I have tears inside," Dirk De Gendt, a local priest who is on the school board, told Reuters. "We don't have words, only deep grief. They were supposed to be back now."     

A teacher and an assistant from St Lambertus were killed along with the bus's two drivers and two other adults. Twenty-four children were being treated in hospital for injuries.

Police said the bus had just joined the highway towards the Swiss town of Sierre after coming down from the resort. After travelling 2 km (1.2 miles) on the road, the bus bumped into the curb and skidded into an emergency siding in the tunnel.     

The front third of the bus was completely torn apart. Many children were trapped in the wreck and had to be freed, said police.     

About 200 police, firefighters, doctors and medics worked through the night, while 12 ambulances and eight helicopters took the injured to hospital. - Reuters