The US Air Force said Monday its fleet of F-22 fighter jets will be allowed back in the air after officials grounded the planes over concerns about the aircraft's oxygen system.

The F-22 Raptors, the most advanced combat aircraft in the world, were barred from flying for four months, a highly unusual move that reflected serious worries over safety.

"We now have enough insight from recent studies and investigations that a return to flight is prudent and appropriate," Air Force chief of staff General Norton Schwartz said in a statement.

"We're managing the risks with our aircrews, and we're continuing to study the F-22's oxygen systems and collect data to improve its performance," said.

Commanders ordered a "stand-down" of the Raptor fleet on May 3 after 12 separate incidents over a three-year period in which pilots reported "hypoxia-like symptoms," the Air Force release said.

In one case, an F-22 reportedly scraped tree tops before landing and the pilot could not remember the incident, indicating a possible lack of oxygen.

Analysts say the Air Force has struggled to pin down the source of the problem with the Raptors despite elaborate safety investigations.

The fleet will undergo "an extensive inspection of the life support systems before returning to flight, with follow-on daily inspections," the Air Force said.

In addition, pilots will use additional protective equipment and undergo physiological tests, it said.

Under the decision, pilots will be allowed to fly at 50,000 feet (15,200 meters). Between January and April, the Raptors were not permitted to venture beyond 25,000 feet.

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

The radar-evading aircraft, designed primarily for dogfights against rival fighter jets, have not been used in the NATO-led air campaign in Libya or the wars in Afghanistan and Iraq.

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