The final F-22 Raptor fighter jet rolls out of the assembly plant during a ceremony marking the occasion at the Lockheed Martin Plant in Marietta, Georgia, in this December 13, 2011, file photo. [Photo/Agencies]

WASHINGTON - US Defense Secretary Leon Panetta has agreed to gradually lift flight restrictions he placed on F-22 Raptor fighter jets, after the Air Force has identified the cause of hypoxia-like symptoms 12 pilots suffered while flying the fifth-generation fighter.

Pentagon Press Secretary George Little made the announcement during a Pentagon news conference, saying Air Force Secretary Michael Donley, Air Force Chief of Staff Norton Schwartz and other Air Force leaders told Panetta on July 20 that they are confident the root cause of the symptoms is the supply of oxygen to pilots.

"The Air Force has taken very prudent measures ... over the past year and a half or so with respect to the F-22. And they have come to the conclusion as to what is causing these hypoxia events," said Little.

In May, Panetta directed the Air Force to limit all F-22 flights to remain near potential landing locations to enable quick recovery and landing should a pilot encounter oxygen deprivation. The secretary also directed the Air Force to expedite the installation of an automatic backup oxygen system in all of the planes.

These actions were in addition to steps the Air Force was already taking to determine the root causes of the hypoxia-like symptoms pilots have experienced.

The Air Force has made two changes that appear to have solved the hypoxia problem. The first was to order pilots not to wear the pressure garment vest during high-altitude missions. It also removed a canister filter from the oxygen delivery system, and that has increased the volume of air flowing to pilots.

Following the Air Force briefing last week, Panetta decided to lift restrictions on the aircraft gradually. Beginning Tuesday, F-22s may resume long-duration flights for deployments, aircraft deliveries and repositioning of aircraft. Little said Panetta has authorized deployment of a squadron of F-22 aircraft to Kadena Air Base, Japan.

"The aircraft will fly to Japan under altitude restrictions using the northern Pacific transit route," Little said. Following completion of the flight to Japan, the Air Force likely will approve most long-duration flights, officials said.