WASHINGTON  - Insufficient oxygen supply caused some F-22 fighter pilots to get dizzy and disoriented when flying the most advanced US warplane, the Pentagon said on Tuesday, signalling at the conclusion of an Air Force study that restrictions placed on F-22 flights would gradually be lifted.

Pentagon spokesman George Little said an Air Force analysis had concluded that symptoms of oxygen deprivation among some pilots of the F-22, built by Lockheed Martin Corp., were caused by problems with the oxygen supply delivered to pilots, not oxygen contamination.

To fix those problems, the Air Force will replace a valve in flight suits that had restricted breathing at high altitude and has increased the oxygen supply to pilots by removing an air filter.

“After receiving assurances that these corrective measures would minimize hypoxia-like events in the F-22, (Defense Secretary Leon Panetta) approved the Air Force planned sequence of actions to remove flight restrictions over time. This process starts today,” Little told reporters.

The Air Force stopped flying the F-22 completely for five months last year. It resumed flights in September, but restrictions have remained on some high-altitude and long-haul flights. Concern about the aircraft flared again this year after two F-22 pilots told CBS’s “60 Minutes” program they had stopped flying the fighter jet due to worry about safety.

Little said the Air Force would also complete other steps designed to make the planes safer, including installation of a back-up oxygen system.

He said altitude restrictions for F-22s could be lifted as early as the fall after a revamped high-altitude garment had been tested and other improvements and studies completed.

Little said the Pentagon would send a squadron of F-22s to a US air base in Japan, after which officials would recommend resuming most long-haul flights by the fighter.

Panetta “believes that pilot safety is paramount,” Little said. “The gradual lifting of restrictions will enable the Air Force to resume normal F-22 operations over time while ensuring the safety of the incredible airmen who fly this critical aircraft.”