The U.S. Federal Aviation Administration has restricted U.S. airlines from flying at or below 30,000 feet (9,100 m) over Iraq, citing a "potentially hazardous situation created by the armed conflict" there, Reuters reported.
The agency issued a special advisory on the change late on Thursday. It previously had restricted flights in Iraqi air space below 20,000 feet (6,100 m).
The FAA also prohibited U.S. airlines from flying in and out of two airports in the Kurdistan region of Iraq, Erbil International and Sulaymaniyah International.
Government forces have been battling Sunni insurgents who have seized parts of Iraq, threatening its survival as a unified country.
The order comes two weeks after Malaysia Airlines flight MH17 was shot down by a missile over a rebel-held area of eastern Ukraine, killing all 298 people on board.
Several non-U.S. carriers have already suspended flights over Iraqi airspace for security reasons.