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 Friday.

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.

In November 2007, the Air Force grounded all F-15 fighters after one of the planes broke apart in flight and crashed.

The planes were not allowed back in the air until March 2008, said Major Chad Steffey.

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

The planes have not been used in the NATO-led air campaign in Libya or the wars in Afghanistan and Iraq.

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