Footage showed the two aircraft on their side in the snow. Photo: BBC

The helicopters hit each other in mid-air during a police exercise. The Bild newspaper said one of the pilots was killed and four people were injured, two of them seriously.

Footage showed the two aircraft on their side in the snow.

Some 400 federal police officers were conducting a football violence training exercise when the crash happened.

An eyewitness told the NTV channel: "Three helicopters were in the air. It was a real snowstorm. Suddenly we heard a bang and someone shouted 'everybody down'. Then there was blood everywhere. Nearby, there was a huge pool of blood."

Pictures showed police and emergency services crawling over the wreck of one of the aircraft, apparently attending to someone inside.

Others cared for a casualty on a stretcher.

A police officer was seen walking from the scene with blood pouring down his face.