The US Fifth Fleet and US aircraft carrier USS Eisenhower in the Gulf of Oman are apparently not allowed to shoot Iranian aircraft without direct permission from Washington D.D. An Iranian Fokker F27 aircraft which on April 21 hovered for 20 minutes, about 900 meters over the aircraft carrier and was no more than 250 meters away, was not fired upon, even though the U.S. military saw the Iranian flight crew gathering intelligence on the Eisenhower and its warship escorts.
The U.S. military did not go public with the information until April 28, 2010.
SOURCE:
DEBKAfile: US Gulf units may not fire on Iranian military without White House say-so