U.S. troops fighting in Afghanistan are experiencing some of the greatest psychological stress and lowest morale in five years of fighting, reports a military study according to USA Today.

"We're an Army that's in uncharted territory here," says Gen. Peter Chiarelli, Army vice chief of staff, according to the report. "We have never fought for this long with an all-volunteer force that's 1% of the population."

Mental health strain was most severe among veterans of three or more deployments, with a third of those showing signs of psychological problems defined as either stress, depression or anxiety, the report obtained by USA Today says.

The research, based on a survey of soldiers and Marines in 2010, also found that the praise the troops have for their unit sergeants has never been higher as the United States approaches the 10th year of its longest war, according to the article.

USA Today writes that the report says decline in individual morale is significant: 46.5 percent of troops said they had medium, high or very high morale, compared with 65.7 percent who said that in 2005. About one in seven soldiers — and one in five Marines — reported high or very high morale.

The report says soldiers and Marines reported more intense fighting than during the surge in Iraq in 2006-07, with 75 percent to 80 percent of those in Afghanistan involved in firefights, according to USA Today.

Half or more of those surveyed said they had killed the enemy, and 75 percent to 80 percent described the death or wounding of a buddy. Half also said that an improvised explosive device detonated within 55 yards while they were on foot patrol. The study's researchers also found evidence of physical wear-and-tear with a third of the force experiencing chronic pain.

Mental health staffing has doubled in Afghanistan since 2009 and troops report better access to this care, though many are so busy fighting "outside the wire" to seek help, the study says.

Read more about Strain on forces from USA Today