Graphic photos published in an American newspaper show US soldiers posing with the mangled bodies of suspected Afghan suicide bombers.
Senior US and NATO officials moved quickly to condemn the troops' behaviour even before the photos were published on Wednesday by the Los Angeles Times, which received them from another soldier.
At a meeting of NATO allies in Brussels, Belgium, Leon Panetta, the US defence secretary, apologised for the actions depicted by the photographs.
"This is war, and I know war is ugly, and is violent. And I know young people sometimes caught up in the moment make some very foolish decisions," he said.
"I am not excusing that."
Panetta said: "My apology is on behalf of the department of defence and the US government ... Again, that behaviour is unacceptable."
He also said he regretted the decision of the Los Angeles Times to publish some of the photos, which he said might trigger retaliatory violence against foreign soldiers stationed in Afghanistan.
Separately, Anders Fogh Rasmussen, NATO secretary-general, condemned the behaviour depicted by the images, saying they "don't in any way represent the principles and values that are the basis for our mission in Afghanistan".
Earlier, General John Allen, the most senior commander of NATO and US forces in Afghanistan, said in a statement that an investigation into the incident was under way.
"The actions of the individuals photographed do not represent the policies of International Security Assistance Force or the US army," he said.
|Pictures suggest 'a breakdown in leadership'
The appearance on the Los Angeles Times website of some of the 18 pictures, taken in 2010, comes at a sensitive time in US-Afghan relations, following release of a video in January that showed four US marines urinating on Afghan insurgent corpses.
The burning of copies of the Quran, the Muslim holy book, at a major NATO airbase also triggered a week of riots that left 30 dead and led to the deaths of six Americans.
And, in March, a US army sergeant went on a night shooting rampage in two southern Afghan villages, killing 17 civilians and prompting Hamid Karzai, Afghanistan's president, to demand foreign soldiers confine themselves to major bases.
Such incidents have complicated US efforts to negotiate a strategic partnership agreement to define its presence once most foreign combat troops pull out by the end of 2014.
Taliban fighters launched suicide attacks in Kabul and three other provinces at the weekend, claiming the assault was launched in retaliation for all three incidents.
In one of the pictures, a paratrooper posed next to an unofficial patch placed beside a body that read "Zombie Hunter", while in another soldiers posed with Afghan police holding the severed legs of a bomber.
Two soldiers in another frame held a dead fighter's hand with the middle finger raised.
Pictures from 2010
The Los Angeles Times said the 82nd Airborne Division soldiers had been at a police station in Afghanistan's Zabol province in February 2010, and revisited several months later.
The pictures were taken on both occasions.
Al Jazeera’s Alan Fisher, reporting from Washington DC, said the US military "didn’t want [the images] published. They asked the LA Times to not put them into the public domain".
He said the US soldier who leaked the images did so because they showed "a breakdown in authority and discipline of soldiers serving in Afghanistan".
The Los Angeles Times defended the distribution of the photos in an article accompanying the photos.
"After careful consideration, we decided that publishing a small but representative selection of the photos would fulfill our obligation to readers to report vigorously and impartially on all aspects of the American mission in Afghanistan," Davan Maharaj, the newspaper's editor, said.