Moving a section of app.config to a separate file

for an easiness of my application I would need to have "runtime" element of app.config in separate file.

Tried something like

<runtime file="runtime.conf" />

and also

<runtime configFile="runtime.conf" />

and in runtime.conf I have:

<runtime>
    <assemblyBinding xmlns="urn:schemas-microsoft-com:asm.v1">  
            .......
    </assemblyBinding>
</runtime>

But it is ignored ...

Is there a way to find out what is wrong.

Of course when I copy runtime section directly into app.config it works.

I somewhere found that when using configFile="..." included file must lay in a subdirectory. Tried it but it also doesn't work...

Seems including is not working for runtime section.