* New restrictions seen as sufficient to guarantee safety
* Pentagon hasn't ruled out grounding F-22 again if necessary
By Phil Stewart and David Alexander
WASHINGTON, May 15 (Reuters) - The Pentagon announced on Tuesday new safety precautions for its F-22 fighter jets - including limiting how far they can fly from airstrips - after pilots experienced symptoms of oxygen deprivation aboard the advanced stealth aircraft.
U.S. Defense Secretary Leon Panetta believes the new precautions on the F-22s, built by Lockheed Martin Corp, are sufficient to guarantee safety. But the Pentagon did not rule out grounding the aircraft again, if necessary.
Panetta "will be receiving regular updates, and all options remain on the table going forward," said Pentagon spokesman George Little.
The F-22s were grounded for over five months last year because of the same issue. But concern over the jet's safety has again taken the spotlight after CBS's "60 Minutes" program aired a report this month in which two pilots said they had stopped flying the fighter due to safety concerns.
The report raised concerns on Capitol Hill, where lawmakers questioned why the Air Force has not yet been able to get to the cause of the problem. A top Air Force official faced tough questioning over the issue at a hearing last week in the Senate.
In the short term, the Pentagon said the decision to limit the distance that jets can fly from landing strips means that other aircraft instead of the F-22 will need to perform long-duration airspace-control flights in Alaska.
The Air Force will also expedite installation in the jets of an automatic backup oxygen system, with the first systems being fitted before the end of the year.
Pentagon spokesman John Kirby said the first reported hypoxia-related event occurred in April 2008, with a total of 12 reported between then and January 2011.
Last week, Lieutenant General Janet Wolfenbarger told a Senate Armed Services subcommittee that the Air Force had implemented over a dozen new procedures to ensure pilot safety in the F-22 and noted that potential problems with oxygen only occurred on 0.1 percent of all flights.
Little said a wide range of potential causes for the problem were still being considered.
"We haven't determined the root cause," he said. "It could be something connected to the oxygen system. It could be other aspects of the aircraft that could contribute to hypoxia-like events, whether it's G forces, the altitude at which the plane flies."
Concerns over the F-22 have also raised questions about whether the new F-35 fighter jets being developed by Lockheed could face similar issues. The stealthy, high-tech F-35 will have many of the capabilities of the F-22 but will not be able to fly as high or as fast.
"I think it's safe to say that everybody in leadership is concerned about this," Kirby said, adding that everyone was "going to work very hard to make sure that the problem gets solved for this aircraft and doesn't get repeated in another."