Deutsche Welle reported seven of those are reported to be seriously injured.
Authorities said the blast came from a taxi being driven alongside the bus by an apparent suicide bomber. A spokesman for the German ISAF said they were investigating who was responsible for the attack, but at least one German official said he received information that it was the al Qaida network.
The German peacekeepers had just finished a six-month tour of duty in Afghanistan and were headed home when the attack occurred.
German Chancellor Gerhard Schroeder called it a "cowardly" act and President Bush expressed his condolences in a telephone call.