[EOB]US Grounds Entire Fleet Of F-22 Fighter Jets

WASHINGTON (AFP): The US Air Force has grounded its entire fleet of F-22 fighters, the most sophisticated combat aircraft in the world, after problems emerged with the plane's oxygen supply, officials said.

The radar-evading F-22 Raptors have been barred from flying since May 3 and Air Force officials could not say when the planes would return to the air.

''The safety of our airmen is paramount and we will take the necessary time to ensure we perform a thorough investigation,'' spokeswoman Captain Jennifer Ferrau told AFP.

The Air Force was probing possible breakdowns in the oxygen supply system for the plane after several pilots reported problems, according to the journal Flight Global.

In one case, an F-22 scraped tree tops before landing and the pilot could not remember the incident, indicating a possible symptom of hypoxia from a lack of air, the magazine reported.

Ferrau said it was too soon to say for certain that the technical problem was related to an onboard oxygen generating system, known as OBOGS.

"We are still working to identify the exact nature of the problem. It is premature to definitively link the current issues to the OBOGS system," she said.

Since January, F-22 pilots have been barred from flying above 25,000 feet (7,600 meters), following the crash of a Raptor jet in Alaska during a training flight.

Grounding an entire fleet of aircraft is a rare step, officials said.