"The order will provide the commander-in-chief additional flexibility should military options be required to protect American lives, citizens and interests in Iraq," said the Pentagon spokesman.
US President Barack Obama has ruled out sending ground troops to help the Iraqi regime against ISIS insurgent advancing on Baghdad but has said other options are under consideration. He will decide in the coming days.
Aircraft carrier Bush will be accompanied by the guided-missile cruiser USS Philippine Sea and the guided-missile destroyer USS Truxtun.
The ships are expected to complete their transit into the Gulf later this evening.