This image provided by the U.S. Navy shows Navy Vice Adm. Tim Giardina in Nov. 11, 2011.

WASHINGTON - The Navy says a three-star admiral was notified Wednesday that he has been relieved of duty as second-in-command at the military organization that oversees all U.S. nuclear forces. He is under investigation in a gambling matter.

The Navy's top spokesman, Rear Adm. John Kirby, said that Vice Adm. Tim Giardina will drop in rank to two-star admiral as a consequence of being removed from his position at U.S. Strategic Command.

Giardina is being reassigned to the Navy staff pending the outcome of a Naval Criminal Investigative Service probe of allegations that he used counterfeit gambling chips at a casino in Iowa, not far from his base in eastern Nebraska.

The removal of such a high-ranking commander of nuclear forces is extremely rare in the U.S. military.