NuGet - NU5131: References declared in the nuspec file are not being packaged

I'm trying to reference two .dll files in a nupkg. I've added the files as references in the nuspec file:

<references><reference file="Project.Modules.ModuleA.dll" /><reference file="Project.Modules.ModuleB.dll" /></references>

When i try to pack, i get the following warning and the package does not include the referenced files.

WARNING: NU5131: References were found in the nuspec, but some reference assemblies were not found in both the nuspec and ref folder. Add the following reference assemblies: - Add Project.Modules.ModuleA.dll to the ref/any/ directory - Add Project.Modules.ModuleB.dll to the ref/any/ directory

Where do i place the files in order to get the package to include the required files?