By Adam Brookes
BBC News, Washington
The US marines have launched a probe into a video posted on the internet that apparently shows a marine singing about the killing of Iraqi civilians.
The US marines have faced controversy in Iraq and at home
A spokesman described the video as "clearly inappropriate" and contrary to the standards of the marines.
The marines said they did not know immediately if the film was genuine.
The lyrics caught on video refer to the shooting of Iraqi civilians, especially children, by insurgents and then how a marine responds, opening fire himself.
The video also refers to Iraqis as "hajis" - a term usually applied to someone who has completed the pilgrimage to Mecca, but commonly used as a term of insult against Iraqis among US troops.
The marines are currently embroiled in controversy over the deaths of 24 civilians in the Iraqi town of Haditha last year.
The four-minute song includes graphic descriptions of killings, real or imagined.
Dressed in a green T-shirt and military style trousers and boots, a man sings: "I grabbed her little sister and put her in front of me.
"As the bullets began to fly, the blood sprayed from between her eyes, and then I laughed maniacally."
Marines spokesman Lt Col Scott Fazekas said: "The video that was posted anonymously is clearly inappropriate and contrary to the high standards expected of all marines."
Asked whether the singer was a real marine, Mr Fazekas said it was impossible to know: "I can't tell. In looking at it, it could be anywhere, it could be anybody."
No decision had been made on whether to launch a formal investigation, the spokesman said.