F-22 Raptor fighter jets have stopped flying indefinitely due to faults in the system that supplies oxygen to pilots.
U.S. Air Force officials issued an order this week to ground the fighter jets "until further notice" after "oxygen system malfunctions" were reported.
"The stand-down provides Air Force officials the opportunity to investigate the reports and ensure crews are able to safely accomplish their missions," the Air Force said.
The F-22 Raptor's On-Board Oxygen Generating System (OBOGS) has been under investigation since one of the jets crashed in November just outside a US Air Force base in Alaska.
Air Combat Command commander Gen. William Fraser has ordered a stand-down of the entire 165-plan fleet, with no indication of how long the fighter jets will be out of service for.
The Raptor is the premier U.S. fighter and has been described as "the world's most advanced fighter jet".
Its cutting-edge "fifth-generation" features includes shapes, materials and propulsion systems designed to make it appear as small as a swallow on enemy radar screens.