According to the Copyright.gov documentation, "copyright can only protect material that is the product of human creativity". The term "author" does exclude non-humans such as animals and AI generated materials.
In the current edition of the Compendium, the Office states that “to qualify as a work of ‘authorship’ a work must be created by a human being” and that it “will not register works produced by a machine or mere mechanical process that operates randomly or automatically without any creative input or intervention from a human author" (Copyright Compendium).
In terms of creativity, the Copyright office considers whether the AI contributions are result of mechanical reproduction or instead of an "authors own mental conception." This depends on how the AI tool operates and how it is used to create the work.
When AI determines the expressive or creative aspects of the work, then the material is not considered a work of human authorship. Then, it has to be "disclaimed in a registration application".
In some instances a work could easily be made of the creativity of a human author as well as contributions made by AI. In those instances, the work will be made up of both copyrightable and non-copyrightable materials. In these cases, copyright will only protect the human-authored aspects of the work, which are “independent of” and do “not affect” the copyright status of the AI-generated material itself.