"It is completely possible that some people did not make it out of the bus, it's unclear where 17 of them are," a police spokeswoman Anne Hoefer, told AFP, adding that 31 people were injured.
The bus carrying 46 passengers and two drivers rammed into the trailer in a traffic jam on the motorway A9 close to the Bavarian town of Stammbach.
The impact shortly after 7 am (local time) sent the bus up in flames.
An emergency team, including firefighters and rescue workers, was in place, while traffic on the motorway was halted in both directions.