It seems like using SourceTree on windows you have to use Putty Agent. OpenSSH keys do not work.
If you have only OpenSSH key:
- click Create or import SSH keys in Tools menu
- in putty key generator select Import key from Conversions menu
- select your private key and click on save private key
You now have PPK file containing your key.
In Source Tree Launch SSH Agent from tools menu and browse to your PPK file.
All operations should now work fine.