The Air Force has grounded its entire fleet of F-22 Raptors, including those in Hawaii, because of concerns about the system that delivers oxygen to pilots aboard the fighter jets, a military spokeswoman said today.

Gen. William Fraser, the head of Air Combat Command at Langley Air Force Base in Virginia, issued the directive earlier this week, said Master Sgt. Pamela Anderson with the command’s public affairs office.

The Hawaii Air National Guard began flying F-22 Raptors last summer in partnership with the active duty Air Force and has nine of the stealth fighters at Joint Base Pearl Harbor-Hickam.

By the end of the year, 20 of the stealthy jets, the Air Force’s most advanced weapons system, are expected to be operating out of Hickam, officials said.

The fleet of 158 fighter jets nationwide is on stand-down because of “hypoxia-like” events reported by some of pilots, Anderson said. Hypoxia is when the body receives too little oxygen.

Last November, an F-22 pilot was killed in Alaska when he lost control of his jet during a training exercise. Since January, the Raptor fleet has been restricted from flying above 25,000 feet because of concerns with the plane’s oxygen supply system.

Capt. Jeffrey Haney was killed when his F-22 crashed 100 miles north of Anchorage. Haney was on a training run with another F-22 to practice “intercepts” when his plane disappeared from ground radar tracking and from communications with the other stealth fighter. The married father of two from Clarklake, Mich., never ejected from the plane.

Anderson said the cause of the Alaska crash has not been determined and cautioned against linking the crash to Fraser’s stand-down order. However, Anderson said pilots reporting what she described as “physiological events” similar to hypoxia was the reason for the general’s order not to fly F-22s.

“The safety of our airmen is paramount, and we will take the necessary time to ensure we perform a thorough investigation,” Anderson said in a prepared response.

Anderson said she did not know how long the stand-down would last. There is a contingency plan in place but she could not immediately provide details.

Hawaii adjutant Maj. Gen. Darryll D.M. Wong, head of 5,500 troops in the Hawaii National Guard, said there have been no reported cases of hypoxia from pilots flying F-22s here.

 The F-22 Raptor was introduced in 2005, but it is unclear if the costly fighter jet made by Lockheed-Martin has ever seen combat.

In March, Gen. Norton Schwartz, the Air Force chief of staff, told lawmakers that he expected the F-22 Raptor to play a prominent role in attacks on Moammar Gadhafi forces in Libya. With its stealth design, the F-22 can evade radar and has advanced engines that allow it to fly at faster-than-sound speeds without using afterburners.

During defense spending discussions in Congress last summer, Defense Secretary Robert Gates was willing to cut the F-22 fighter jet program as too costly. When a spending bill was passed, there was no money in it for more F-22s and production was capped.

As the Hawaii Air National Guard builds up its stock of F-22s, the Montana Air National Guard has been fulfilling the air defense role for Hawaii with F-15 Eagle fighters, Wong said.