By ROBERT BURNS, AP National Security Writer

WASHINGTON (AP) — The Air Force on Friday temporarily stopped flying F-35 fighter jets at Luke Air Force Base in Arizona in response to a series of incidents in which pilots reported symptoms of hypoxia, or oxygen deprivation.

Air Force spokesman Capt. Mark Graff said the temporary halt followed five separate in-flight incidents since May 2. Graff said in each case, the airplane's backup oxygen system worked as designed and the pilot was able to land the plane safely.

"The Air Force takes these physiological incidents seriously, and our focus is on the safety and well-being of our pilots," said Brig. Gen. Brook Leonard, 56th Fighter Wing commander at Luke. "We are taking the necessary steps to find the root cause of these incidents."

The cancellation is limited to F-35s at Luke, and the Air Force intends to resume flight operations there on Monday, Graff said. He initially said flying would resume Saturday but later said he had been mistaken. Luke normally does not conduct F-35 flight operations over the weekend, he said. F-35 operations at five other U.S. bases are not affected, he said.

Graff said the Air Force has established an "action team" of engineers, maintenance specialists and aeromedical specialists to study the five Luke incidents.