Defense Secretary Leon Panetta called on US troops to tighten up discipline and display "integrity" after a series of public relations disasters that he said could play into the hands of America's enemies.

Panetta, echoing a message from Army and Marine Corps leaders, on Friday said the misconduct related only to a small percentage of the force but that digital technology magnified any incident, posing a threat to the military's image.

"These days, it takes only seconds, seconds, for a picture, a photo to suddenly become an international headline," he told hundreds of troops at Fort Benning, Georgia.

"And those headlines can impact the mission that we're engaged in, they can put your fellow service members at risk, they can hurt morale, they can damage our standing in the world and they can cost lives."

The Pentagon chief appealed to the troops to uphold the highest standards of conduct.

"I need every one of you, every one of you, and all of your fellow service members, to always display the strongest character, the greatest discipline, and the utmost integrity in everything you do," he said in a televised speech.

Panetta's trip to Fort Benning followed a string of damaging incidents, including a video of Marines urinating on Taliban corpses, photos of soldiers posing with body parts and the burning of Korans that sparked deadly riots in Afghanistan.

The high-profile public relations setbacks showed "a lack of judgment, a lack of professionalism, and a lack of leadership," he said.

The Pentagon had previously portrayed the misconduct as isolated cases of bad behavior. But comments from military chiefs and Panetta signal the top brass believes discipline has to be bolstered across the armed forces.

Senior military officers have long voiced worries about the effect of years of protracted ground wars on the all-volunteer force, amid a spike in suicides, divorce, mental health problems, and substance abuse among troops.

The incidents in Afghanistan have not only aggravated relations with Kabul but threatened to undermine already declining support for the war in Afghanistan among Americans, according to former officers and analysts.

In his speech, Panetta said the US military's power ultimately depended not on weaponry but on the quality and behavior of people in uniform.

It is the "character and the standards that each of you bring to the battle that makes us strong," he said.

The commandant of the US Marine Corps, General James Amos, recently issued a bluntly-worded "white letter" to the entire chain of command on what he called a lack of discipline.

"We are allowing our standards to erode," Amos wrote in the letter, published by the Marine Corps Times. "A number of recent widely publicized incidents have brought discredit on the Marine Corps and reverberated at the strategic level."

Amos has since embarked on a tour of Marine bases across the country to hammer home his point, and will head to installations on the West Coast later this month, his spokesman said.

"He's going to be traveling around the Marine Corps to address these issues personally," his spokesman Lieutenant Colonel Joe Plenzler told AFP.

"You have to bear in mind the Marine Corps and Army have borne the brunt of a lot of hard fighting on the ground in a fairly morally bruising environment for the last decade plus," he added.

"This is one effort to kind of grab everybody by the face masks, especially the leadership... and set that expectation, and just make sure we're all moving in the right direction."