Fire service spokesman Sven Gerling said his initial information was that two helicopters had crashed, but he had no further details on the accident.
Federal Police spokesman Frank Brochert confirmed there "was an incident during an exercise" and that emergency crews were on the scene. He had no details about the accident.
More top news
Kieran Trippier has received his first call-up to the England squad after impressive form for Spurs.
The prime minister said the public should remain vigilant following a meeting of the Government's emergency committee Cobra.
The Queen said the Manchester Arena suicide bombing was "very wicked" she visited injured victims at the city's children's hospital.