Lockheed Martin has launched an offensive to combat complaints from pilots who have refused to fly its F-22s over concerns about oxygen deprivation while in the cockpit.

The company took its campaign to the skies - er, Twitter - to try to combat growing negative publicity about its Raptors.

The Air Force has been looking into about a dozen unexplained incidents related to hypoxia, or oxygen deficiency, with pilots but has been unable to pinpoint the cause, Air Combat Command has said.

Some pilots have come forward to say they won't get in the F-22s until the problem is solved.

Pilots began experiencing problems about four years ago.

“For some reason, the onboard oxygen generating system and the environmental control system that feeds it may be inputting some contaminant,” Gen. Gregory Martin, a retired Air Force veteran, told CNN affiliate WAVY in Virginia.

For a while, the problem was the subject of only a spattering of media reports, but Lockheed Martin went on the offensive (or defensive, depending whom you ask) by launching a Twitter campaign praising the fleet as "60 Minutes" aired a segment on the problems with the Raptors and interviewed decorated pilots who were refusing to fly them.

Gen. Mike Hostage of Air Combat Command recently spoke about the issue, which has plagued the fleet since problems with the F-22s' oxygen supply system were reported in 2008.

The jets have been grounded to examine the problem, but in September 2011, the Raptors were again cleared and allowed to fly. In January 2011, the jets were limited to altitudes under 25,000 feet during an investigation into a November 2010 crash. Flying above that altitude could cause a pilot to black out from lack of oxygen and lose control.

The Air Force has made sure to add emergency oxygen deployment handles should a pilot encounter any issues.

"We are diligently pursuing a variety of hypotheses to try and understand and characterize the exact circumstances we've been experiencing," Hostage said.

As the "60 Minutes" feature aired, Lockheed Martin tweeted about the impressive speeds and missions that no other planes but the F-22s were able to claim. But it also got a few pithy responses to the public relations campaign.

@LockheedMartin Pity the pilots don't want to fly them though... —

Damo ✌ (@ThatDamonGuy) May 07, 2012