The US Air Force has had to ground dozens of F-22 fighter jets for the second time this year after concerns a pilot suffered a lack of oxygen in the cockpit, officers said Monday.

Commanders at a base in Virginia and in Alaska ordered a "pause" in flights for the world's most expensive and advanced fighter aircraft as a safety precaution, an Air Force spokesman said.

The decision came after an incident last week in which a pilot at Joint Base Langley-Eustis in Virginia suffered "hypoxia-like" symptoms in mid-flight, Lieutenant-Colonel Richard Johnson told AFP.

The announcement came only a month after the Air Force grounded the entire Raptor fleet from May through mid-September -- an extraordinary step -- to allow engineers to check for possible problems with the plane's oxygen supply.

Analysts say the precise source of the problem remains a mystery despite elaborate tests and safety measures.

The fleet was cleared to return to the air last month without a clear explanation behind a spate of incidents in which pilots appeared to suffer from a lack of oxygen.

"Part of our protocol is to allow units to pause operations whenever they need to analyze information collected from flight operations to ensure safety. That is what is happening at Langley at the moment, and we support that decision," Johnson said in an emailed statement.

About 30 F-22 Raptors are based at the Langley base in Virginia, which President Barack Obama visited last week.

At Joint Base Elmendorf-Richardson in Alaska, senior officers lifted the temporary flight ban on Monday after having ordered the planes grounded last Thursday, he said.

The Air Force has been reluctant to discuss the problem in any detail, particularly the circumstances of about a dozen incidents affecting F-22 pilots over a three-year period.

At a cost of nearly $150 million for each plane, the F-22 Raptor is designed mainly for dogfights against rival fighter jets and the radar-evading aircraft were not used in the NATO-led air campaign over Libya.

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

Some US lawmakers and defense contractors lobbied for years to fund a larger F-22 fleet but former defense secretary Robert Gates succeeded in defeating their effort, saying there was no urgent need to build more of the sophisticated aircraft.