A large fire broke out Thursday in a popular forest in western Berlin following an explosion in a police munitions storage site, as a new heatwave was due to envelop Germany over the day.

Firefighters were still unable to begin putting out the flames as the affected area of 15,000 square metres (161,500 square feet) included a storage site for police ammunition.

"There are still explosions" at the storage area neighbouring Grunewald forest, said a spokesman for Berlin's firefighter service.

"The situation is unpredictable. It's burning uncontrollably in the forest," he added.

Officials are building a security cordon to allow firefighters to begin extinguishing the flames from a distance of about a kilometre from the ammunition storage zone.

A police helicopter was circling the area, as authorities appealed for the public to keep away.

Temperatures are expected to climb to as high as 40 degrees Celsius (104 degrees Fahrenheit) across parts of Germany. In Berlin, they are predicted to reach 38C.

Brandenburg, the region surrounding Berlin, has for days been battling forest fires.