The army sent in a tank aimed at evacuating munitions at the affected storage site as well as remote-controlled de-mining robots, while drones were circling the air to help assess the emergency.
The situation is “extremely extraordinary with munitions,” said Berlin fire brigade spokesman Thomas Kirstein, adding that it was “under control and there was no danger for Berliners.”
The fire was however expected to last for some time, he said.
Firefighters called to the site in the middle of the night were confronted with intermittent blasts that sent debris flying and hindered their work.
They have so far been able to begin tackling only two of the four hotspots across the affected area of 15,000 square metres (161,500 square feet), as explosions at the munitions store were still rocking the area.
More than 140 firefighters were on the scene. No one has been hurt by the fires, which came on a day when a new heatwave hit Germany.
“So far, there are no injured people and no-one has been affected by an evacuation,” Kirstein said at midday.
The below tweet shows explosions early on Thursday morning in Grunewald. The clip was taken from the roof of the Corbusierhaus in the Westend area by building technician Michael von Rein.
— rbb 88.8 (@rbb88acht) August 4, 2022
Officials have built a security cordon to allow firefighters to start working around a kilometre from the ammunition storage zone.
The store holds munitions uncovered by police, but also unexploded World War 2-era ordnance which is regularly dug up during construction works.
Police said they were investigating what set off the fire.
Authorities appealed for the public to avoid the forest, popular with both locals and tourists, as several regional rail lines have been halted.
But authorities said no firefighting choppers were available as they were already in use to calm forest fires in eastern Germany.
Earlier in the day the spokesman for the Berlin fire brigade described the tense conditions.
He said: “The situation is unpredictable. It’s burning uncontrollably in the forest.”
Disruption to transport
As a result of the ongoing fire in Berlin’s largest forest, regional and S-Bahn services to the west were disrupted on Thursday.
The S-Bahn line S7 was only running from Berlin to Grunewald, a railway spokesperson told DPA.
Regional trains running between Berlin-Wannsee and Berlin-Friedrichstraße, including the RE1, RE7, RB21 and RB22 were also disrupted.
It’s currently not clear to what extent long-distance traffic is affected by the fire.
#Explosions being clearly heard in this video taken shortly after a big fire broke out in the #Grunewald forest in #Berlin where a police ammunition site seems to be burning as well. Warning to keep windows shut. https://t.co/9G8AHg3Rdb
— Katerina Alexandridi (@kalexandridi) August 4, 2022
The Avus motorway between Spanischer Allee and Hüttenweg is closed in both directions, as are Kronprinzessinnenweg and Havelchaussee, the Berlin traffic centre said.
Authorities warned the public about the fire on warning apps and called for people to avoid the area. They advised nearby residents to keep windows and doors closed, and said ventilation and air conditioning systems should be turned off.
The German capital is rarely hit by forest fires, even though its 29,000 hectares of forests make it one of the greenest cities in the world.
Brandenburg, the region surrounding Berlin, as well as parts of eastern Germany have for days been battling forest fires.
Scientists say climate change is making heatwaves around the world more frequent and more intense, which increases the risk of fires.
Temperatures are expected to climb to as high as 40C across parts of Germany. In Berlin, they are predicted to reach around 36-38C before it becomes cooler on Friday.