Dec 11 (Reuters) - U.S. custodian bank State Street Corp is exploring options for its asset management business, including a merger with a competitor, as it seeks to gain scale, Bloomberg News reported on Friday, citing people familiar with the matter. The bank has been informally working with an adviser to review strategic alternatives for the unit, known as State Street Global Advisors, according to the report bloom.bg/37Y8Irp. Boston-based State Street Corp has evaluated possible combinations with the asset management operations of rivals including Invesco Ltd and UBS Group AG, Bloomberg reported. State Street was not immediately available for comment.