A humpback whale stranded on Germany's Baltic Sea coast freed itself overnight after days of rescue efforts, biologist Robert Marc Lehmann said on Friday.

The whale had been stuck in shallow waters off Timmendorfer Strand since early on Monday, drawing heavy media attention.

Lehmann said the whale had been able to swim into deeper water through a channel dug out by a floating excavator. The biologist had snorkelled out to the animal the previous day and tried to guide it through the trench.

Lehmann said the crucial thing now was for the 12- to 15-metre marine mammal to remain in open water and, if possible, make its way to the North Sea. It was still not safe, he stressed, saying its release from the sandbank was not yet a rescue, but only a small step in the right direction.

The animal would only be home once it reached the Atlantic, Lehmann added.