WASHINGTON - Facing a mysterious safety problem with the Air Force's most-prized stealth fighter, Defense Secretary Leon E. 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 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.