The Federal Aviation Administration banned all US civilian flights over Iraq on Friday, just hours after air strikes order by Washington on Islamist fighters.
In a Notice to Airmen, the FAA cited "the potentially hazardous situation created by the armed conflict" between Islamic State militants and Iraqi security forces "and their allies" as the reason for the indefinite ban.
The ban extends to "all US air carriers and commercial operators," as well as US-licensed pilots unless they are flying US-registered aircraft for a foreign operator, the NOTAM states.
It was not clear how many US air carriers are affected by the ban.
Iraq lies on the flight path for several non-US long-haul carriers operating between Europe, the Middle East and Asia.
Turkish Airlines, one of the key foreign carriers flying to Iraq, said Friday it had halted flights to the main city of Iraq's Kurdish region for security reasons amid an offensive by Islamist jihadists.
"Our flights to Arbil are being cancelled for security reasons until further notice," the airline said in a statement.
Abu Dhabi's Etihad Airways on Thursday also announced a suspension of flights to Arbil, while Britain has urged citizens living in parts of Kurdistan to leave.
US warplanes earlier Friday bombed positions held by Islamic State insurgents who have advanced to take swathes of northern Iraq.