Firefighters have dodged flying debris and the threat of explosions to battle a fire at an ammunitions dump in western Berlin.

The blaze in a munitions storage site in Berlin’s west also sparked a fire in the city’s Grunewald forest on Thursday, on one of the hottest summer days in the German capital.

More than 100 firefighters used water cannons and an army robot to try to contain the fire, which had triggered explosions and was still spreading, officials said.

“What we have here is an ordnance site. So ordnance is stored and also fireworks and other objects that can also explode, detonate,” Berlin fire department spokesman James Klein told German news website DW.

“There are different items stored there, so there can be smaller and larger explosions. We are optimistic, because the heavy ordnance also located there, is stored accordingly, cooled and in steel containers, so that we can protect them.”

The fire service set up a perimeter zone 1000 metres from the centre of the blaze, which it said had spread across 15,000 square metres of Grunewald, a forested area bordered by upscale villas and swimming lakes as well as the sealed-off ammunitions dump.

Firefighters had been unable to reach the source of the blaze due to explosions at the bomb disposal site. They were using the robot to obtain images of the area.

They also laid hoses through the forest from nearby rivers and lakes and an armoured vehicle supplied by the Bundeswehr cut a fire break through the forest at the southern edge of the perimeter zone.

“Everything is prepared at the borders of the perimeter. That is where we want to stop it,” a spokesperson for the fire service told Reuters.

Berlin Mayor Franziska Giffey labelled the fire “unprecedented in the post-war history of Berlin”.

Berlin frequently carries out bomb disposal operations as explosives are still being discovered from World War II.

The Grunewald site is used to defuse such old munitions as well as weapons and fireworks, a fire department spokesperson said.

It has been in operation since 1950 and has fire alarm systems and a firebreak that is several metres wide, police said on Twitter.

The explosives stored there were being continuously sprinkled with water.

The fire service said it was unclear what had caused the fire but that several buildings at the bomb disposal site were ablaze by the time firefighters arrived.

No residential areas were affected by the fire and no evacuations had been ordered.

It came as Berlin endured a heatwave. Weather service DWD forecast said Thursday was one of the hottest days of the year with temperatures expected to reach between 34-38 degrees.

Germany has also been tackling wildfires at its border with Czech Republic for more than a week.

-with AAP