Any suggestion that our practices are fraudulent is outrageous and false," the insurer said.

The new civil fraud investigation is examining UnitedHealth's practices for recording diagnoses that trigger extra payments to its Medicare Advantage plans, including at physician groups the insurance giant owns, the report added. DoJ and UnitedHealth did not immediately respond to a Reuters request for comment.

It extends a tumultuous period for UnitedHealth marked by the killing of a top executive, a costly cyberattack against its subsidiary and high medical costs.