WASHINGTON -- About 300,000 US military veterans of the wars in Iraq and Afghanistan currently suffer from post-traumatic stress disorder (PTSD) or major depression, an independent study released Thursday estimates.

The study by a team at the RAND Corporation also concludes that about 320,000 veterans of those conflicts experienced a "probable" traumatic brain injury (TBI) during deployment, but the long-term impact on mental health is unclear.

The findings were extrapolated from a survey of 1,926 recently returned members from across the military services. The sample was designed to represent the 1.6 million troops who have deployed to Iraq and Afghanistan since 2002.

"Among those who met diagnostic criteria for PTSD or major depression, only 53 percent had seen a physician or mental health provider to seek help for a mental health problem in the past 12 months," the study said.

"The gap in care was even higher for TBI: 57 percent of those who reported experiencing a probable TBI were never evaluated by a physician for a brain injury," it said.

Troops' exposure to the impact of roadside explosions in Iraq and Afghanistan is believed to be behind the growing incidence of traumatic brain injury.

The military only has recently begun to recognize TBI as one of the "invisible wounds" of the war, and the RAND researchers said the condition is still not well understood, particularly in the more common milder cases such as concussions.

The survey found that 19.5 percent of the returning soldiers reported experiencing a "probable TBI" during deployment, or about 320,000 service-members since 2002 if the sample is representative.

The survey found that 18.5 percent of all returning service-members meet criteria for either post-traumatic stress disorder (PTSD) or major depression.

That finding led researchers to conclude that about 300,000 service-members or veterans currently suffer from those long-lasting mental health conditions.