US Defense Secretary Leon Panetta has ordered the Air Force to restrict flights of its most advanced fighter jet, the F-22, due to safety concerns voiced by some pilots, officials said Tuesday.

The F-22 Raptor has been plagued by an unresolved engineering problem at least since 2008 that has seen pilots complaining of dizzy spells and black-outs due to a lack of oxygen.

Under Panetta's decision the F-22 will no longer be conducting longer-range flights and would instead stay within reach of runways to ensure a pilot could land in an emergency, Pentagon spokesman George Little told reporters.

He said that "effective immediately, all F-22 flights will remain within the proximity of potential landing locations, to enable quick recovery and landing should a pilot encounter unanticipated physiological conditions during flight."

Little said the move meant that "long-duration airspace control flights" out of Alaska would be carried out by other aircraft.

Panetta also called on the Air Force to "expedite" the installation of a back-up oxygen system in the F-22 planes and to provide a monthly progress report on efforts to get to the bottom of the undiagnosed technical problem.

Officials said the first back-up systems would be installed by December.

Panetta's decision followed a CBS "60 minutes" television report in which two F-22 pilots said they had refused to keep flying the warplane because of fears over hypoxia symptoms. The two pilots have sought "whistleblower" legal protections through a Republican lawmaker.

The F-22 Raptor was grounded last year for four months after a spate of incidents with pilots saying they had passed out or suffered a lack of oxygen. The plane was cleared for flying in September 2011 but engineers are still trying to solve what they believe may be a problem with the jet's oxygen supply.

The F-22, manufactured by Lockheed Martin, influenced the design of another new fighter still being developed, the F-35 Joint Strike Fighter, also produced by Lockheed Martin.

But officials said there was no sign the F-35 shared the F-22's oxygen problem.

"I think it's safe to say that everybody in leadership is concerned about this, even in the defense industry environment," Pentagon spokesman Captain John Kirby told the same briefing.

"But we are all going to work very hard to make sure that the problem gets solved for this aircraft and doesn't get repeated in another."