At least 22 children -- most aged around 12 -- have died in a horrific bus crash in a tunnel in Switzerland.

A further six adults were killed and 24 children have also been injured after the bus hit a curb and slammed into a concrete wall in a mountain tunnel near Sierre late on Tuesday.

The bus was carrying 52 mostly Belgian school-children back to the towns of Lommel and Heverlee from a skiing trip, the Belgian foreign ministry said.

Some of the victim's families will be flown to Switzerland on two specially prepared Belgian military aircraft, while others are due to meet at the effected schools to comfort each other.

A survivor who broke both legs and an arm has described what happened, according to the newspaper Lesoir.

It was very dark, and I heard a huge shock. All the seats went flying and I found myself between two seats, she told her father, according to the Huffington Post.

Speaking from Herleeve, the archbishop of Belgium Andre Joseph Leonard said parents were struggling to cope with news of the tragedy.

There is this terrible fear and uncertainty. There are about eight about whom we don't know what happened, leaving their parents in terrible fear, he said, according to the Asoociated Press.

Belgian Prime Minister Elio Di Rupo added it was a tragic day for all of Belgium, the BBC reported.

The children were travelling as part of a three-bus convoy, hired by a Christian group.

It is believed at least two of the bus drivers were among the dead.

More than 200 people, including several helicopters, battled throughout the night to rescue those trapped in the wreckage.

The injured were flown to nearby hospitals in Lausanne, Bern and other Swiss cities.

Speaking to the BBC, Swiss journalist Ruth Seeholzer said the two-lane tunnel had been clear of traffic when the accident occurred.

According to Swiss prosecutors the bus was new and was equipped with safety belts, while Belgian Transport Minister Melchior Wathelet added that the bus company had an excellent reputation.

The drivers had arrived the night before and had rested during the day before departure. It seems that the law on driving and rest periods has been respected, he said, according to the BBC.

Belgium's ambassador to Switzerland, Jan Luykx, is at the crash site, waiting for the arrival of the parents.

This tragedy will hit the whole of Belgium. The magnitude of the accident is difficult to take in. For the moment I am concentrating on the practical aspects.

The emotional side will come when we meet the families, he said.