That means U.S. naval forces are operating in the same waters as an organization the United States considers a major supporter of terrorist activity.
Adm. Michael Mullen, chairman of the Joint Chiefs of Staff, made the disclosure Wednesday at the Army War College in Carlisle, Pennsylvania, where he was answering questions from military students.
Afterward, in a written statement, the U.S. Navy Fifth Fleet in Bahrain said, "Based on activities observed in the Arabian Gulf over the past several months, it appears the Iranian navy has shifted its patrol areas to the Strait of Hormuz and Gulf of Oman — leaving the IRGC navy to provide the primary Iranian naval presence in the Arabian Gulf."
The move is of concern to the U.S. Navy, which has long viewed the IRGC’s forces as more antagonistic than Iran’s regular navy.
Mullen said Iran made a "strategic decision" in recent months to "essentially give the entire Gulf to the IRGC over the next four or five years."
"That’s a big deal, because I think part of the leading-edge challenge with Iran is the IRGC specifically," Mullen said.
For the past several months, IRGC forces have occupied a sunken barge and crane near Iraqi oil terminals at the northern end of the Persian Gulf. The IRGC is using the site as an observation post for the area, which is patrolled regularly by U.S. and coalition naval forces.
Mullen’s comments reflect the chairman’s concern about not just Iran’s nuclear program, but also its arms shipments to Iraq and Afghanistan and statements against Israel, a senior U.S. Navy official said.
The United States has long said it believes the IRGC is behind those arms shipments, but it has stopped short of saying the central government of Iran is responsible for those actions.
IRGC forces earlier this year seized a group of British sailors at the northern end of the Gulf and held them for several days. The British had been conducting a boarding of a merchant vessel, as part of an approved coalition operation in the Gulf.