LOS ANGELES - In response to growing concern about problems with its F-22 Raptor fighter jet, the Air Force revealed that it had imposed new safety restrictions to protect its pilots.

The announcement came as Sen. Mark R. Warner (D., Va.) and Rep. Adam Kinzinger (R., Ill.) on Friday requested additional information from the secretary of the Air Force to further determine the scope of safety concerns raised by several pilots of the world's most expensive fighter jet, designed and built by Lockheed Martin Corp.

The Air Force has acknowledged that some of the nation's top aviators are refusing to fly the radar-evading F-22, a fighter jet with problems with its oxygen systems that have plagued the fleet for four years.

"The health and safety of our pilots - all of our pilots - is the utmost priority," said Brig. Gen. Daniel O. Wyman, an Air Force command surgeon. "Our operational flight surgeons and medical staff interact with our pilots on a daily basis, and mission No. 1 is their health and safety."

The comments, posted on the Air Force's website, were meant to address the growing attention directed at the safety of the F-22. Concerns have grown in recent months as no clear explanations have emerged for 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 F-22 is considered the most advanced fighter jet in the world. It entered military service in 2005, and the Air Force received the last of its order of 188 planes 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 costs 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, resulting in 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 F-22 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.

The Air Force lifted the grounding in September. When that happened, Wyman revealed, the Air Force put all F-22 pilots through retraining so they would know their own specific hypoxia symptoms. It also affixed a device to pilots' fingers that measures the amount of oxygen in the blood while they are in the cockpit.

Last Sunday, two F-22 pilots appeared with Kinzinger on CBS's 60 Minutes to discuss reasons why they refused to fly the jet.

At the risk of significant reprimand - or even discharge from the Air Force - Virginia Air National Guard Capt. Joshua Wilson and Maj. Jeremy Gordon said they would not fly the F-22 until the oxygen problems were solved.