By ROBERT BURNS, AP National Security Writer

WASHINGTON (AP) — Facing a mysterious safety problem with the Air Force's most-prized stealth fighter, Defense Secretary Leon Panetta on Tuesday ordered new flight restrictions on the F-22 and summoned help from Navy and NASA experts.

Panetta endorsed Air Force efforts to figure out why some F-22 pilots have experienced dizziness and other symptoms of an oxygen shortage while flying, but his personal intervention signaled a new urgency. A secretary of defense does not normally get involved in a service-specific safety issue unless it is of great concern.

The Air Force grounded its F-22s for four months last year because of the oxygen-deficit problem, and now some pilots are refusing to fly them. An Air Force advisory panel headed by a retired Air Force general studied the problem for seven months and reported in March that it could not pinpoint the root cause. It endorsed a plan keeping the aircraft flying, however, with pilots using special sensors, filters and other safety precautions.

Panetta was briefed on the problem last Friday, just days after a CBS "60 Minutes" report featured two F-22 pilots who said that during some flights they and other pilots have experienced oxygen deprivation, disorientation and other problems. They cited safety concerns as well as the potential for long-term personal health issues.

Asked why Panetta was acting now, Navy Capt. John Kirby, a Pentagon spokesman, said the defense chief has been aware of the F-22 problem "for quite some time." In light of the recent deployment of several F-22s to the Persian Gulf and because of pilots' expressions of alarm, Panetta chose to "dive a little more deeply into the issue."

In a letter to Air Force Secretary Michael Donley, Panetta ordered that F-22 flights remain "within proximity of potential landing locations" so that pilots can land quickly in the event they experience an oxygen-deficit problem. Kirby said the specifics of those flight restrictions are to be set by individual F-22 pilots and commanders.

Panetta also told Donley to accelerate the installation of an automatic backup oxygen system in each F-22. The first of those is to be ready for use by December, Kirby said.

And the Pentagon chief ordered the Air Force to call on the expertise of the Navy and NASA in pursuit of a solution.

Panetta's actions have no immediate effect on U.S. combat operations, since the F-22 is not in Afghanistan. But Panetta said the plane would give up long-distance air patrol missions in Alaska until the planes have an automatic backup oxygen system installed or until Panetta agrees the F-22 can resume those flights. Other aircraft will perform those missions in the meantime.

Panetta's chief spokesman, George Little, told reporters that Panetta supports the Air Force's efforts to get to the bottom of the problem.

"However, the safety of our pilots remains his first and foremost concern," Little said.

Little did not rule out Panetta taking additional measures. Asked whether Panetta considered grounding the fleet again, Little said Tuesday's less drastic moves are "the prudent course of action at this time," adding that Panetta will keep a close eye on the situation, "and all options remain on the table going forward."

In a conference call with reporters, Sen. Mark Warner, D-Va., and Rep. Adam Kinzinger, R-Ill., said they were briefed by the Air Force and told that the number of pilots who came forward with complaints has risen from two to nine. Warner called Panetta's action a "step in the right direction" but said questions still remain.

"This is a confidence issue that has to be addressed fully and transparently by the Air Force," Warner said.

The F-22 Raptor, which has never flown in combat, recently deployed to the United Arab Emirates for what the Pentagon called routine partnering with a Middle East ally. Little, the spokesman, told reporters that Panetta's order to impose new flight restrictions would not affect flight operations during the UAE deployment.

The plane, conceived during the Cold War as a leap-ahead technology that could penetrate the most advanced air defenses, is seen by some as an overly expensive luxury not critical to fighting current conflicts. The fleet of 187 F-22s — the last of which was fielded just two weeks ago — cost an average of $190 million each.

Panetta's predecessor as Pentagon chief, Robert Gates, persuaded Congress to cap production of the F-22 earlier than originally planned. He saw it as primarily of use against a "near-peer" military competitor like China, noting that the plane did not fly a single combat mission during a decade of war in Iraq and Afghanistan.

With its stealth design, the F-22 is built to evade radar and has advanced engines that allow it to fly at faster-than-sound speeds without using afterburners. It manufacturer, Lockheed Martin, describes the plane as "the only fighter capable of simultaneously conducting air-to-air and air-to-ground combat missions with near impunity."

The fleet of 170 F-22s is stationed at six U.S. bases: Joint Base Elmendorf-Richardson, Alaska: Joint Base Pearl Harbor-Hickam, Hawaii; Joint Base Langley-Eustis, Va.; Nellis Air Force Base, Nev.; Holloman Air Force Base, N.M.; and Tyndall Air Force Base, Fla.

F-22 pilots are trained at Tyndall. Flight testing is at Edwards Air force Base, Calif., and operational testing and tactics development is performed at Nellis.