Can some PBIs be moved to the product backlog while the sprint is half completed?

Suppose a sprint is currently in session, and half-way through the sprint, the team realises that it won't be able to complete some of the PBIs, should the PBIs be moved back to the product backlog? Or should they be kept in the sprint as it is and wait for the sprint to get over? What is the ideal activity? Should it be discussed with the PO?

Posted By - ramesh