A malfunctioning weapons system on a Danish navy ship forced the closure of a portion of the country's air and sea traffic on Thursday.
The missile launcher on board the frigate Niels Juel was activated and cannot currently be switched off, the Danish Defence Ministry said.
The vessel is stationed at the Korsør naval base.
The technical failure occurred during a mandatory test on the system, the ministry said. Specialists are on their way to solve the problem.
Until the problem is fixed, there is a risk that a missile could be mistakenly fired and fly up to several kilometres from the frigate.
The danger zone stretches between 5 and 7 kilometres from the naval base. Marine traffic has been told to stay away from the area and the airspace above has been closed off.