About 9 per cent of United States troops who have performed combat duty in Iraq and Afghanistan report experiencing post-traumatic stress disorder (PTSD), according to a study led by Defence Department researchers.

The study, published in the British Medical Journal, provides the latest estimate of this mental disorder among US troops returning from war.

It tracked PTSD among 50,000 US troops, looking at differences in rates among those who saw combat, those deployed in non-combat roles and those who did not deploy.

The study finds that some troops develop it after wartime trauma like suffering wounds or witnessing others being hurt, or it can appears months or years after a traumatic event.

"The unpredictability and intensity of urban combat, constant risk of roadside bombs, multiple and prolonged tours, and complex problems of differentiating enemies from allies can leave many troops with high stress levels and possible lasting health consequences," the researchers wrote.