Two commuter trains crashed head-on in southern Germany on Tuesday, killing at least eight people and injuring more than 100, in one of the country's deadliest rail accidents in years.

Hundreds of rescuers raced to pull passengers from the wreckage in a wooded area near Bad Aibling, a spa town about 60 kilometres southeast of Munich.

Several carriages were derailed, with blue, yellow and silver metal debris left strewn around the crash site next to a river in the southern state of Bavaria.

"We have eight dead on the trains," said police spokesman Juergen Thaimeier.

A total of 126 people were injured, including 15 critically and another 40 with serious injuries, said national rail operator Deutsche Bahn.

The two train drivers and a conductor were among the dead, local broadcaster Bayerische Rundfunk reported.

At least 100 firemen were deployed in the rescue operation, which was complicated because the forest crash site was difficult to access, said Thaimeier.

Rescuers focused on the impact area of the trains, using electric saws to cut through the mangled wreckage.

Rescue workers from nearby Austria were also on site, rolling news channel NTV said.

Underlining the difficulty of the emergency operation, mountain rescuer Joerg Becker told NTV: "The terrain is not only difficult to access but the large number of injured also requires a massive coordination effort between so many rescue and aid groups."

'Deeply shocked'  

The "tragic accident occurred on the single-track route between Rosenheim and Holzkirchen this morning shortly after 7:00 am," regional rail company Meridian, a subsidiary of the French group Transdev, said in a statement.

The cause of the accident was not immediately clear.

Rainer Scharf, a Bavarian police officer, said that "given the severity of the accident, we believe the two regional trains collided head-on at low speed".

About a dozen helicopters were deployed, with television footage showing them waiting in a clearing outside the forest, from where rescuers were emerging with stretchers carrying the injured.

A journalist for Bayerischer Rundfunk reported that rescuers were climbing on the wreckage and pulling people out.

The rail route and two nearby roads were closed to traffic.

"The accident is an enormous shock for us," said Bernd Rosenbusch, who heads the Bavarian rail company BOB that operates trains on the route.

"We will do everything to help travellers, their relatives and our employees."

Christian Schreyer, chief executive of Transdev, said: "We are deeply shocked and stunned that something like this could have happened. Our thoughts are with the victims and relatives of the victims".

The accident is believed to be Germany's first deadly train crash since April 2012, when three people were killed and 13 injured in a collision between two regional trains in the western city of Offenbach.

The country's deadliest post-war accident happened in 1998, when a high-speed ICE train linking Munich and Hamburg derailed, killing 101 people and injuring 88 at the northern town of Eschede.