A large fire has broken out in one of Berlin’s biggest city forests, 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 of the year.

Around 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 the Second World War, 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.”

He said it was not clear what triggered the first explosions at the ammunition dump early on Thursday.

The site belongs to the Berlin police, and controlled blasts are carried out there twice a year for several days at a time.

It was created in 1950, and before Thursday’s explosions around 25 metric tons of fireworks, Second World War ammunition and other explosive ordnance was stored there, the German news agency dpa reported.

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 blaze as emergency forces had to keep 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 judge the situation.

“The plan is to get a new situation assessment from the blast site — initially from the air,” Mr Kirstein said, adding that they hoped to access the area with armoured 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 taskforces to help extinguish the flames, including special forces from the 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 blaze could spread due to the dry conditions of the forest and expected temperatures of up to 38C.