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BERLIN (AP) — A large fire broke out in one of Berlin’s biggest city forests on Thursday morning, triggered by several explosions that took place at an ammunition dump inside the forest on what is forecast to be one of the hottest days so far this year.

More than 100 firefighters were battling the blaze in the German capital’s Grunewald forest in the west of the city, German news agency dpa reported.

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 is stored and controlled explosions are carried out.

“The situation is dangerous,” Thomas Kirstein from the Berlin fire department told reporters. “The fire is not under control, the forest is burning uncontrollably.”

Kirstein said it was not clear what had triggered the first explosions at the ammunition dump early Thursday. The site belongs to the Berlin police. It was created in 1950, and before Thursday’s explosions, around 25 metric tons of fireworks, World War II ammunition and other explosive ordnance was stored there, the German news agency dpa reported. Controlled blasts are scheduled there twice a year for several days at a time.

Kirstein 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 as emergency forces had to keep a distance of a kilometer (more than half a mile) from the flames. Firefighters on the scene were pouring water on the forest outside the banned area around the ammunition dump trying to prevent the blaze from spreading further.

They were also planning to get aerial images by flying a drone over the ammunition dump to better be able to judge the situation.

“The plan is to get a new situation assessment from the blast site — initially from the air,” Kirstein said, adding that they hoped to access the area with armored vehicles later in the day. According to first estimates an area of 1.5 hectares was burning on Thursday morning.

Berlin authorities called for additional task forces to help extinguish the flames, including special forces from the German army. Water cannons 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 degrees Celsius (100 Fahrenheit).