Iranian sailor monitors a Revolutionary Guards missile

WASHINGTON (CNN) -- Iran's Islamic Revolutionary Guards Corps has taken command of Iranian naval operations in the Persian Gulf, the U.S. military has revealed.

An Iranian sailor monitors a Revolutionary Guards missile test in the Persian Gulf in 2006.

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.