The Air Force has lifted altitude restrictions on its F-35A joint strike fighters at an Arizona base, which were imposed after a string of oxygen deprivation incidents among pilots this year.

A cause of five incidents in May and June at Luke Air Force Base is still not known, the service said on Wednesday, but it is allowing the pilots to again fly the high-tech fighter jets above 25,000 feet.

"We have learned a lot over the past two months and while we have yet to identify a singular cause we have reduced potential causes for labored breathing, carbon monoxide ingestion, and refined our procedures and training," said Brig. Gen. Brook Leonard, the base's 56th Fighter Wing commander.

The F-35s at Luke comprise about 25 percent of the total military-wide fleet and resumed flying below 25,000 feet in June after an 11-day grounding over the oxygen issues. Such incidents can cause sickness, hyperventilation, anxiety and panic in pilots.

The F-35A is the Air Force variant of the tri-service aircraft, and takes off and lands on conventional runways.

The Air Force said it has been working with the F-35 Joint Program Office and experts to figure out the cause behind the string of mysterious incidents. The Navy has also grappled with spikes in oxygen deprivation incidents among pilots of its F/A-18A-D model Hornets and T-45 training jets, which were grounded for nearly three months.

Leonard said he has "great confidence" in the F-35 aircraft and that close monitoring will continue.