Distraught parents flew to Switzerland on Wednesday after a bus carrying a Belgian school party home from a ski trip crashed into the wall of a Swiss tunnel, killing 22 children and six others.

Some of the parents, gathering at the primary schools in Belgium before boarding a military aircraft, did not know whether their children were dead or alive.

There's no news, simply no news, said one red-eyed father.

The bus was carrying 52 people, mostly children aged about 12, when it crashed in the Swiss canton of Valais at 9:15 p.m.(2015 GMT).

Twenty-eight people were killed, including 22 children. Of the 24 injured, three were in a coma. Most pupils were from the towns of Lommel and Heverlee in Belgium's Dutch-speaking Flanders region.

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. It was later towed away from the scene.

Police were alerted to the accident by images on surveillance cameras in the tunnel.

It gives you chills down the spine. Witnessing such a drama involving children takes away my voice, police spokesman Jean-Marie Bornet told Swiss television.

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.

A teacher from St Lambertus were killed along with the bus's two drivers and three other adults.

Teachers covered boards outside the school and the school gates with their pupils' drawings - many rainbows, flowers and references to Mister Frank, the sixth-grade teacher who died.

I hope we will wake up soon and that this nightmare will be over, one pupil wrote.

The bus had not been driving for long after heading down from the resort and had only been on the valley highway towards the Swiss town of Sierre for 2 km (1.2 miles) when it hit the curb and veered head-on into an emergency siding in a tunnel.

No other vehicle appeared to be involved. 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 at the scene, while 12 ambulances and eight helicopters took the injured to hospital.

Help arrived quite quickly, said Eric van Maldarin, a Belgian driver who happened to be in the tunnel at the time of the crash and who helped injured children off the bus.

The first children who came out weren't hurt very badly but the others were a lot more, he told Reuters.

BLACK DAY FOR BELGIUM

It is a black day for all of Belgium, a visibly moved Belgian Prime Minister Elio Di Rupo told a news conference. He later arrived at Geneva airport and was whisked by helicopter to the site of the crash.

Belgium plans to hold a national day of mourning.

Swiss parliamentarians stood for a minute's silence.

I can imagine what it means for the families who sent their children on holiday and expected them back. And these children aren't coming back, said President Eveline Widmer-Schlumpf.

Swiss prosecutors are investigating whether the cause of the accident was a technical problem or human error. The speed limit in the tunnel was 100 km per hour.

The bus owned by Belgian company Toptours was returning to Belgium from a skiing holiday camp in Val d'Anniviers, a resort in the Valais Alps that border France.

Belgian transport minister Melchior Wathlelet said Toptours had a solid reputation and the driver had arrived at the resort a day before the trip, according to the rules, and that the bus had passed a mechanic's test five months ago.

Ten Dutch children were on the bus, nine who live in Belgium and one in the Netherlands. There was also one German and a Polish person on the bus, the Belgian government said.

The trip was coordinated by Intersoc, a Belgian social organisation which organises inexpensive trips for 85,000 members of Christian social and health funds a year. Two other buses on the tour returned to Belgium safely.

Switzerland's mountain regions have a history of deadly crashes. Tuesday's was the country's worst since 1982 when 39 German tourists were killed on a railway crossing when a train hit their bus. Twelve people were killed and 15 injured when a bus crashed into a ravine in the Valais region in 2005.

We have seen a number of catastrophes in Valais but we have never experienced such a large number of young deaths. We are really totally upset, Jean-Pierre Deslarzes, head of the rescue operation, told Swiss television.

The bodies of the dead were taken to a funeral home in the town of Sion. The busy highway that is a main access route to many ski resorts was open again. A 2005 inspection of the tunnel gave it a good grade but a medium risk rating.

Last month, a British teacher was killed and more than 20 people hurt in northern France after a coach crashed while bringing school children home from a skiing trip in Italy.

(Additional reporting by Katharina Bart and Andrew Thompson in Zurich; Ben Deighton, Claire Davenport and Robert-Jan Bartunek in Brussels, Robin Bleeker in Sierre; Geert de Clercq in Paris; Writing by Emma Thomasson; Editing by Robin Pomeroy and Alessandra Rizzo)