A piece of forest caught fire on a former military site in Münster (Darmstadt-Dieburg district).
The fire broke out on Saturday afternoon, the fire department said.
The extinguishing work was difficult because there could still be ammunition on the site of the former ammunition plant.
Some areas of the site are therefore not accessible to the emergency services.
Six firefighters suffered minor injuries while fighting the fire.
Fire brigades from the Darmstadt-Dieburg, Offenbach and Bergstraße districts are on site to fight the fire.
A fire-fighting helicopter from Rhineland-Palatinate is also in action.
"But we assume that more emergency services will have to be added," said a spokesman for the fire department.
The extinguishing work could therefore take days.
No further details were given about the cause of the fire or injuries.