The U.S. military says newly diagnosed cases of post-traumatic stress disorder among American troops sent to Iraq and Afghanistan climbed nearly 50 percent last year, bringing a five-year total to nearly 40,000.

Officials Tuesday released figures that showed Marines and Army soldiers were most affected. These are the forces bearing the brunt of combat in Iraq and Afghanistan.

The Army alone reported more than 10,000 new cases of post-traumatic stress disorder in 2007, while the Marine Corps had more than 2,100 cases. The military says the figures reflect cases it has tracked between 2003 and 2007.

Army Surgeon General Lieutenant General Eric Schoomaker says the larger number of diagnoses in recent years partly reflects greater awareness and tracking of the disorder by the U.S. military. But he says increased exposure of troops to combat is a factor.

Experts have said symptoms increase as soldiers return to combat for multiple tours of duty.

The National Institute of Mental Health in the United States identifies the disorder as a health condition that can develop after exposure to a terrifying event or ordeal in which grave physical harm occurred or was threatened. Symptoms can include sleep difficulties, irritability and outbursts of anger.

Some information for this report was provided by AP and Reuters.