Photos:: Two commuter trains crash head-on in southern Germany

Two commuter trains crashed head-on Tuesday morning in southern Germany, killing at least nine people and injuring about 150, slamming into each other on a curve without braking after an automatic safety system apparently failed to stop them, the transport minister said.

Rescue personel work at the site of a train accident near Bad Aibling, Germany.

The first rescue units were on the scene within three minutes of receiving emergency calls, but with a river on one side and a forest on the other, it took hours to reach some of the injured in the wreckage.

"This is the biggest accident we have had in years in this region, and we have many emergency doctors, ambulances and helicopters on the scene," police spokesman Stefan Sonntag said.

The cause of the accident was not immediately clear.

German rail operator Deutsche Bahn said safety systems on the stretch had been checked as recently as last week, but Transport Minister Alexander Dobrindt suggested a system designed to automatically brake trains if they accidentally ended up on the same track didn't seem to have functioned properly.

Black boxes from both trains had been recovered and are now being analyzed as part of the investigation to determine what went wrong.