The area affected by the flames has grown considerably, to over 150 hectares, said a spokeswoman for the city of Jüterbog on Saturday evening. Earlier in the day, it was said that 45 hectares were burning.
"There was stronger wind today than on previous days and higher temperatures," the fire brigade's head of operations Rico Walentin told dpa.
As a result, he said, the fire in the woods by Jüterbog, 60 kilometres south-west of the capital Berlin spread faster on Saturday. "If the weather stays like this, we expect it to spread further," Walentin added.
The nearest villages, however, were still far away, he said.
While the fire brigade is trying to combat the fire on the ground, the ammunition load in the area is hampering their efforts.
"It won't be done tomorrow," said Raimund Engel, the state's forest fire protection officer. He said that fire fighters were also having to respond to forest fires in other regions, including near Bad Liebenwerda in southern Brandenburg.
Engel said he expected higher temperatures on Sunday, with no rain in sight.
Near Jüterbog, large clouds of smoke could be seen from afar on Saturday.
Fire crews were struggling to reach the location because of the danger of old ammunition exploding.
"There is frustration," Walentin said.
Two fire-fighting planes that repeatedly dropped water on the burning area on Friday were no longer in action as their effect was insufficient, according to Walentin. He said more planes would have been needed.
Ten fire engines were instead deployed to pump water from the roadside.
"We have to see if the wind plays in our favour," the fire chief said, hoping for calm and above all rain.
The fire brigade is trying to stop the fire at firebreaks, 30-metre aisles in the forest, said the head of the city's public order office, Christiane Lindner-Klopsch. They were also using drones to assess the extent of the fire and a quad bike to help with reconnaissance.
Walentin said the firefighters were also considering using fire-fighting and recovery tanks from a private provider but it was not clear how much this would help.
"There are munitions all over the site and we don't know where," he said. A lot of dead wood was a further problem, saying it was not easy to procedure through the forest given that the ground was "like a big game of mikado."
The forest fire alert level for the state of Brandenburg remains high.
The affected forest fire area belongs to a large area owned by the Brandenburg Natural Landscapes Foundation, which protects the former military training areas for nature conservation.
The foundation's managing director Andreas Meißner, lamented that the "heart" of the wilderness area was affected, adding that he was saddened to see the fires return to the region.
There have already been fires in the area in 2018 and 2019, he said, resulting in a forest fire protection system being developed in the area.