What should I do if I am asked to review a paper that should have been reviewed by an ethics committee, but was not? Assuming the paper is missing the statement that it was reviewed, and the paper clearly states that data was collected from human subjects.
I think it's quite a leap to jump from the fact that there is no such statement to the conclusion that it has not been reviewed. You should ask for clarification on that point in your review, rather than outright rejecting it.
Also, there are different rules in different countries, and the notion of an IRB might not exist, or the procedure may go by a different name. In our department, for instance, a formal process of ethical approval was only implemented a couple of years ago.
So, short answer: ask what ethical procedure was followed, and how that relates to the obligations in the country and/or university the researchers and the participants are based.
Your first step should be to contact the editor with your concerns. It may be that the research falls under one of the exceptions for which IRB approval is not needed. See, www.eandireview.com, for example, though there are others. Both NHS and NIH list some exceptions.
However, even if an exception applies but you think that the research was, in fact, unethically conducted, then you may refuse to review. It is your duty, actually, but you should't do so silently. In fact, it might be best to point out your ethical concerns in a review.
I'll also note that retroactive approval is generally against the rules: explicitly disallowed. So, sending the paper back to get a review that isn't already in place is not an option if it were required in the first place. The purpose of the IRB is to protect human subjects, not to check a box. So approval needs to come first.
But the problem may simply be one of presentation in the paper as noted by Roland, and a simple edit would correct it. But it is up to the editor to communicate that to the authors.
I'd review it as normal, but add a note (to the comitee, perhaps also the author) stating concerns about the need to involve the ethics comitee, with a short but clear explanation as to why I think it is required.
Rationale: I might be mistaken, perhaps it is being reviewed by said comitee in parallel, ...