U.S. Defense Secretary Leon Panetta has called on the Air Force to "expedite" the installation of a back-up oxygen system in the F-22, its most advanced fighter jet and to provide a monthly progress report on efforts to resolve the undiagnosed technical problem.

The first back-up systems would be installed by December, U.S. officials said Tuesday.

Panetta ordered the Air Force to restrict flights of the F-22, after some pilots complained of dizzy spells and blackouts, the officials said.

Since 2008, officials say at least a dozen F-22 pilots have reported suffering a lack of oxygen but engineers have yet to figure out how to fix the problem.

The move followed a report on CBS's "60 Minutes" program on May 6 in which two F-22 pilots said they had refused to keep flying the warplane because of safety fears. The two pilots have sought "whistleblower" legal status through a Republican lawmaker.

The announcement on the radar-evading F-22 marks the latest setback for the aircraft, the most expensive in the U.S. fleet at $143 million per plane. Touted as the most sophisticated fighter in the world, the F-22 has yet to be deployed in combat, and its cost overruns became a long-running political controversy until the program was curtailed.

The fleet of 187 Raptors 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 suspect may be a problem with the jet's oxygen supply.

The F-22 flies at a higher altitude than other jets, above 50,000 feet, and relies solely on pressurized oxygen instead of a mixture of oxygen under pressure and air in the cockpit, according to the Air Force.

The plane is also faster and more agile than older fighters, with the pilot facing more gravitational forces than in other planes, officials say. (AFP)