Belgium has started a day of national mourning for the 28 people who died in a school bus crash in Switzerland, with solemn music filling the airwaves and official buildings lined with flags at half mast.

As a chilly morning fog lifted, the nation of 11 million began preparing for a minute of silence at 11am to mark Tuesday's crash, which ended the lives of 22 children returning from their "snow classes" in the Alps - a traditional rite of passage in Belgium from childhood to the teenage years.

Trains are expected to halt along with other traffic for the minute's tribute.

Swiss police in Valais said the bodies were being repatriated on three Belgian military planes Friday.

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