Around 100 firefighters were on site to control the blaze in the Grunewald forest in the west of the city, German news agency DPA reported.

The fire has so far affected some 15,000 square meters of the forest, the city firefighting unit said, while a one-kilometre perimeter has been set up as the situation remains high-risk.

The fire was spreading quickly, and massive explosions could be heard from the site where old ammunition from World War II, fireworks and explosive ordnance are stored, and controlled explosions are carried out.

"The situation is dangerous," Thomas Kirstein from the Berlin fire department told reporters. He called on residents to stay away from the forest.

Due to flying debris and the threat of further explosions, the fire department said it had not yet begun to systematically extinguish the fire.

Berlin authorities called for additional task forces to help extinguish the flames, including special forces from the German army. Water cannons, helicopters and special evacuation tanks were also due on the scene.

The commuter train service to the city's west was partially interrupted, and one of the city's most important highways, the Avus, was closed.

Homes were not directly threatened by the flames, but the fire department warned that the fire could further spread due to the dry conditions of the forest and the exceptional heat that was expected on Thursday with temperatures of up to 38°C.

The Berlin region is suffering from a particularly severe drought as well.

A large forest fire in July burned more than 850 hectares in southern Brandenburg, the region surrounding the capital. A little further south, a fire in the "Bohemian Switzerland" national park, which started in the Czech Republic and spread to Germany, is still keeping firefighters busy.