A fire that broke out on Friday evening at the Eagle's Nest mountain retreat in southern Germany that was once used by Nazi dictator Adolf Hitler erupted again overnight to Sunday in the almost destroyed building, Bavarian police said.

A police officer said flames there burnt again at "room height" after smoldering embers provoked a flare-up in what the officer said was a common occurrence in the case of such conflagrations.

The renewed fire triggered another deployment of firefighters to the site, known as Kehlsteinhaus in German, at the top of the Obersalzberg mountain.

The site, formerly used by Hitler to receive diplomats and other foreign guests, has been a restaurant since 1952 but was closed on Friday when the fire broke out. No one was injured.

Police arrested a 33-year-old German man suspected of having lit the fire, with an initial psychiatric assessement indicating mental illness.

The building was largely destroyed by the blaze, which caused millions of euros of damage.