The US Air Force has allowed dozens of F-22 fighter jets back in the air after commanders briefly grounded the planes at two bases amid safety concerns, officials said Tuesday.

After a pilot suffered a lack of oxygen in the cockpit last week, senior officers at bases in Virginia and in Alaska ordered a "pause" in flights for the sophisticated F-22 Raptors, the world's most expensive combat aircraft.

At Joint Base Elmendorf-Richardson in Alaska, the temporary ban was lifted on Monday and the commander at Joint Base Langley-Eustis in Virginia ended the grounding on Tuesday.

"The 1st Fighter Wing has resumed flying operations at Langley AFB" at 8:00 am (1200 GMT), said Miles Brown, spokesman for the Virginia air force base.

Air Force engineers will continue to collect data from the F-22 flights to ensure safety, he added.

It was the second time this year that F-22 fighter jets were grounded.

The Air Force stood down the entire Raptor fleet from May through mid-September -- a highly unusual move -- to allow engineers to check for possible problems with the plane's oxygen supply.

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

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

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 per plane, the F-22 Raptor is designed mainly for dogfights against rival fighter jets. 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.