US Defence Secretary Leon Panetta has condemned as "utterly deplorable" a video that purports to depict four US Marines urinating on the corpses of Taliban fighters.
He said such behaviour is "entirely inappropriate for members of the United States military" and those responsible will be held accountable.
Mr Panetta said he had ordered the Marine Corps and Marine General John Allen, the top commander of the Nato-led forces in Afghanistan, to investigate.
The Marine Corps said earlier it would investigate the YouTube video but had not yet verified its origin or authenticity. The case has been referred to the Naval Criminal Investigative Service, the US Navy's worldwide law enforcement arm.
The video, posted on the internet, shows men in Marine combat gear, standing in a semi-circle over three bodies.
It is not clear whether the dead were Taliban or civilians or someone else. The title on the posting called them Taliban insurgents but it was unclear who added that title, Marine Corps officials in Washington said. The reaction from Afghanistan was angry.
Afghan president Hamid Karzai condemned the video as "completely inhumane", while the Afghan defence ministry called it "shocking".
The Taliban issued a statement accusing US forces of committing numerous "indignities" against the Afghan people.
"First they killed the Afghans with mortars, and they then urinated on their bodies," Taliban spokesman Zabiullah Mujahid said. "We strongly condemn this inhumane action by the wild American soldiers."
A US official later said the Marine Corps has identified at least two of the four Marines in the internet video. A Marine official said that the four were members of the 3rd Battalion, 2nd Marines, which returned to its home base in North Carolina last autumn after a tour in Afghanistan.