U.S. Defense Secretary Leon Panetta on Tuesday announced new measures to restrict the flight missions by the 5th-generation F-22 Raptor fighters, after pilots continued complaining about hypoxia-like symptoms during flight.Panetta has directed the U.S. Air Force to take three new measures in order to ensure the safety of F-22 fighter pilots, Pentagon spokesman George Little told reporters at a news briefing. The measures include the installation of an automatic back-up oxygen system in all F-22 fighters, suspension of long-duration air space control flights by the F-22 fighters, especially in Alaska, and providing a monthly progress report on fixing the problem. "Secretary Panetta believes the department must do everythingpossible to ensure pilot safety and minimize flight risks. He will continue to closely monitor the Air Force’s efforts to enhance the safety of this very important aircraft,” Little said.Asked why Panetta did not order to ground all F-22 fighter jets until the problem is completely fixed, Little said that "all options remain on the table” as the defense chief will receive regular updates. But he added that, in terms of the deployment of F-22 fighters in southwest Asia, Washington will not change the plan as "we believe that we can safely continue that deployment given the geography of the region.”Little stressed that, despite the problems reported, the F-22 fighters remain the most advanced U.S. fighter aircraft, whose capabilities are important to maintaining the U.S. air superiority and national security objectives, whether it’s protecting the U.S. air space or deploying overseas as part of deterrence and engagement efforts.Panetta supports the measures taken so far by the Air Force to pursue all plausible hypotheses and determine the root causes of the hypoxia-like symptoms experienced by F-22 pilots, he added. The U.S. Air Force has been examining a total of 25 reports by F-22 fighter pilots who experienced hypoxia-like symptoms during flight, or oxygen deficiency.