The Federal Aviation Administration is restricting U.S. airlines from flying at or below 30,000 feet (9,145 metres) over Iraq because of what it calls "the potentially hazardous situation created by the armed conflict" there.
The agency already had restricted flights in Iraqi air space to a 20,000-foot (6,095-meter) minimum.
The order issued Thursday night follows the downing two weeks ago of Malaysia Airlines Flight 17 by a missile as it flew over rebel-held territory in eastern Ukraine.
Iraq also has been in turmoil as militants have fought government forces. Amid fears that militants had weapons capable of shooting down planes, several European carriers already have developed alternate flights plans for Iraq.
The FAA also is prohibiting U.S. carriers from flying in and out of two Iraqi airports, Erbil International and Sulaymaniyah International.