Two F-22 pilots say they have stopped flying the US Air Force's most advanced fighter jet because of safety fears over the aircraft's oxygen system, according to a CBS television "60 Minutes" report.

The F-22 Raptor was grounded last year after a spate of incidents with pilots suffering dizzy spells and blackouts in the air. The plane was cleared for flying in September 2011 but engineers are still trying to solve what appears to be a problem with the jet's oxygen supply.

The pilots, Major Jeremy Gordon and Captain Josh Wilson, told the "60 Minutes" program they stopped flying in January, citing safety concerns over a lack of oxygen.

Asked if he believes the jet is safe, Gordon said: "I'm not comfortable answering that question. I'm not comfortable flying in the F-22 right now," according to excerpts from the interview, due to be aired on Sunday.

"The onset of (hypoxia) is insidious. Some pilots will go the entire mission, land and not know anything went wrong," Gordon is quoted as saying.

The two pilots, who both served in the Iraq war, have sought legal protection as "whistleblowers" from a Republican lawmaker from Illinois, Adam Kinzinger.

The pilots were from the Air National Guard, officials said.

The airmen's dismay over safety will add to the controversy surrounding the aircraft, as some lawmakers and analysts have long questioned the costly plane's value.

The Air Force declined to comment on the television report but a spokesman said it views safety as a top priority.

"While the F-22 program has encountered challenges, the Air Force remains committed not only to their resolution but also to unparalleled dedication to flight safety," Lieutenant Colonel John Dorrian told AFP.

After the plane was grounded in May last year, a scientific inquiry could not trace the problem to an engineering fault, he said.

But the Air Force has taken precautions, including adjusting a backup system, "to minimize crew risk," he said.

The F-22 jet, the most expensive warplane ever built by the Pentagon, is designed mainly for dogfights against rival fighters. The radar-evading planes were never used in the NATO-led air campaign over Libya or the wars in Iraq and Afghanistan.

The Air Force has more than 160 F-22 Raptors in its fleet and plans to build a total of 187.