Unethical procedure in methods I am currently reviewing a paper for a journal and there's a section in the 'Methods' that may raise a questionable ethical concern on animal handling. The paper has a potential contribution to the field, however, there's this procedure that harms the animal in the study. I am thinking of rejecting the paper and raise a correspondence with the authors. Will the rejection decision a good decision for the paper?
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.