Unethical procedure in methods
You said, "MAY raise a questionable ethical concern". So, you might possibly misunderstand what they did, perhaps because they might have explained things poorly. I think that you should give the authors the benefit of doubt and let them defend themselves: recommend a major revision, acknowledging the potential value of their study, but make it clear that you insist that they document that they have followed ethical procedures in the method of concern. If they cannot provide satisfactory evidence of ethical methods, then you would be justified to reject the article for ethical problems.

And, as many other comments indicate, you should clearly explain your concerns to the editor.