German police say at least 30 children have been injured, some of them seriously, after a bus they were traveling in overturned on a southern highway.

A Bavarian police spokesman says the driver appeared to have lost control of the bus during a sudden hailstorm near the town of Freising on Wednesday afternoon.

Ulrich Poepsel said police sent "anything with a siren" to the site of the accident including several helicopters that ferried the injured children to nearby hospitals.

He says the bus was registered in the German town of Dachau and was taking children home from a holiday trip.

Poepsel said the overturned bus caused a second crash involving several vehicles for which casualty figures were still unclear.