[_delirium]

I believe the answer is yes, you need to first register before filing suit. If the offending work continues to be published, you can register and then sue.

An exception is foreign works, whose owners can file a suit without first registering, since treaty obligations don't allow the U.S. to subject foreign holders to "formalities". However foreign holders are limited to recovering "actual damages" in such suits, and need to register to be eligible for per-copy statutory damages.