WASHINGTON (Reuters) - About 300,000 U.S. troops returning from Iraq and Afghanistan suffer symptoms of post-traumatic stress disorder or depression, but about half receive no care, an independent study said on Thursday.

The study by the RAND Corp. also estimated that another 320,000 troops have sustained a possible traumatic brain injury during deployment. But researchers could not say how many of those cases were serious or required treatment.

Billed as the first large-scale nongovernmental survey of its kind, the study found that stress disorder and depression afflict 18.5 percent of the more than 1.5 million U.S. forces who have deployed to the two war zones.

The numbers are roughly in line with previous studies. A February assessment by the U.S. Army that showed 17.9 percent of U.S. troops in Iraq and Afghanistan suffering from acute stress, depression or anxiety in 2007, down from 19.1 percent in 2006.

But the 500-page RAND study, based in part on interviews with more than 1,900 soldiers, sailors and Marines, also said that only half of troops suffering debilities receive care. And in half of those cases, the care is only minimally adequate.

"There is a major health crisis facing those men and women who have served our nation in Iraq and Afghanistan," said Terri Tanielian, a RAND researcher who helped head the study.

"Unless they receive appropriate and effective care for these mental health conditions, there will be long-term consequences for them and for the nation."

FEAR OF STIGMA

The study said many service members do not seek treatment because they fear the stigma associated with psychological problems could harm their careers.

Post-traumatic stress disorder, or PTSD, can result from wartime trauma such as suffering wounds or witnessing others being hurt. Symptoms include irritability or outbursts of anger, sleep difficulties, trouble concentrating, extreme vigilance and an exaggerated startle response.

RAND recommended that the Pentagon create a way for service members to receive mental health service confidentially and monitor the quality of care.

Army Col. Loree Sutton, director of the U.S. Defense Center of Excellence for Psychological Health and Traumatic Brain Injury, welcomed the study.

She was concerned at the finding that only about half of those who sought help received "minimally adequate" treatment and said it would spur the military to try harder to recruit more mental health specialists.

The Army wants to hire 275 civilian mental health professionals but a tight labor market and difficulties getting civilians into war zones has slowed the effort, officials say.

RAND, a private research organization, estimated that stress and depression among returning soldiers cost $6.2 billion in the two years following deployment, mainly due to lost productivity, medical costs and a higher risk of suicide. (Additional reporting by Andrew Gray; Editing by Alan Elsner and Will Dunham)