LOS ANGELES - Some of the nation's top aviators are refusing to fly the radar-evading F-22 Raptor, a fighter jet with ongoing problems with the oxygen systems that have plagued the fleet for four years.

At the risk of significant reprimand - or even discharge from the Air Force - fighter pilots are turning down the opportunity to climb into the cockpit of the F-22, the world's most expensive fighter jet.

The Air Force did not reveal how many of its 200 F-22 pilots, who are stationed at seven military bases across the country, declined their assignment orders. But current and former Air Force officials say it's an extremely rare occurrence.

"It's shocking to me as a fighter pilot and former commander of Air Combat Command that a pilot would decline to get into that airplane," said retired four-star Gen. Richard E. Hawley, a former F-15 fighter pilot and air combat commander at Langley Air Force Base in Hampton, Va.

He said he couldn't remember one specific incident in his 35-year career in which a fighter pilot declined his assignment.

Concern about the safety of the F-22 has grown in recent months as reports about problems with its oxygen systems have offered no clear explanations why pilots are reporting hypoxia-like symptoms in the air. Hypoxia is a condition that can bring on nausea, headaches, fatigue or blackouts when the body is deprived of oxygen.

The Air Force's handling of the investigation is being closely watched throughout the military and in Congress.

The F-22, designed and built by Lockheed Martin Corp., is considered the most advanced fighter jet in the world. It entered service in 2005, and the Air Force is set to receive the last of its order of 188 planes later this month.

The plane can reach supersonic speeds without using afterburners, enabling it to fly faster and farther. It's also packed with cutting-edge radar and sensors, enabling a pilot to identify, track and shoot an enemy aircraft before that craft can detect the F-22. The Air Force says the aircraft is essential to maintain air dominance around the world.

According to the Air Force, each of the sleek, diamond-winged aircraft costs $143 million. Counting upgrades and research and development costs, the U.S. Government Accountability Office estimates each F-22 cost taxpayers $412 million.

While other warplanes in the U.S. arsenal have been used to pummel targets in Iraq, Afghanistan and Libya, the Air Force's F-22s have sat largely idle - used only in test missions. Even so, throughout the jet's development, F-22 pilots have been in seven serious crashes, including two fatalities.

Over the years, F-22 pilots have reported dozens of incidents in which the jet's systems weren't feeding them enough oxygen, causing wooziness. This issue led to the grounding of the entire fleet last year for nearly five months, but even after the grounding was lifted the Air Force said investigators could not find a "smoking gun."

Since the F-22 returned to service in September, the Air Force said, there have been 11 incidents in which F-22 pilots reported hypoxia-like symptoms.

"Obviously it's a very sensitive thing because we are trying to ensure that the community fully understands all that we're doing to try to get to a solution," said Gen. Mike Hostage, commander of Air Combat Command, who told reporters in Washington on Monday about the pilots declining to fly, according to The Associated Press.

The Air Force doesn't have specific details on numbers and locations of pilots who have refused to fly the F-22, said Maj. Brandon Lingle, an Air Force spokesman.

"We are generally aware of a small number of pilots who have expressed reservations about flying the F-22, and each of those cases will be handled individually through established processes," he said.

There are Air Force rules that say a fear of flying, "whether expressed in general terms or limited to a particular aircraft, is a professional dereliction that carries significant consequences," Lingle said.

Air Force officials maintain that F-22s are safe to fly. New precautions for pilots have been put into place, such as: wearing a device that measures the amount of oxygen in the blood, taking blood samples and watching over pulmonary functions.

Currently, there are hundreds of people in the Air Force, other governmental agencies such as NASA and in the defense industry working to pinpoint causes of the problem, Lingle said.

Lockheed, for instance, has about 20 people working "to bring this physiological issue to resolution to alleviate any concerns," spokeswoman Alison Orne said.

Neither Lockheed nor the Air Force has an estimated date when the problem will be resolved. The Air Force is looking into whether pilots are not getting enough oxygen or whether toxins are not being adequately filtered out of the oxygen supply, Lingle said.

"We are deliberately stepping through a thorough analysis of both hypotheses and we're confident we're getting closer to identifying a root cause or root causes," Lingle said.

The F-22's issues have also popped up on Congress' radar, said John Noonan, an aide to Rep. Howard P. "Buck" McKeon R-Calif., chairman of the House Armed Services Committee.

"The Air Force is rigorously examining the issues with the F-22, and Chairman McKeon is closely monitoring that investigation," Noonan said.