The screams of children following a coach crash in Switzerland left rescuers traumatised.

First responders to the crash in the canton of Valais, which killed 26 people, including 22 children, were thrown into "state of shock" by the terribel carnage at the crash scene.

"The first thing we heard on arriving at the scene was the screams of the children - It's almost impossible to describe," said Alain Rittemer, chief of emergency services in Valais.

He said it took two hours for rescuers to pick their way through the wreckage of the coach, which hit the side of a tunnel on the A9 on Tuesday.

"You could imagine that the children whom we found were our own," said Mr Rittemer. "This was a vision which we were not used to seeing, and I've been doing this job for 20 years.

"Our main aim was to get to the children, most of whom could not move because they were trapped in the piles of scrap metal.

"That said, we did not need to speak. We only needed to look in their eyes and hold their hands."

The coach was filled with 52 schoolchildren and staff from the Saint Lambertus school in Belgium who were returning from a skiing trip in the Swiss Alps.

Grieving relatives and friends of the victims returned to the crash scene to pay their respects, while tributes were left at the school.

Investigations into the cause of the crash are still ongoing. At a press conference officers revealed that they were investigating possible technical problems with the coach itself, a health problem that caused the driver to lose control or simple human error.