A posting on the internet included a photo of what appeared to be a uniformed US soldier seated with his hands tied behind his back, and a gun barrel pointed at his head.
"God willing," the statement read, "we will behead him if our female and male prisoners are not released from US prisons within the maximum period of 72 hours from the time this statement has been released."
The soldier is seated in front of a black banner emblazoned with the Islamic profession of faith, "There is no god but God and Mohamed is his prophet." A statement posted with the picture suggested the group was holding other soldiers.
"Our mujahedin heroes of Iraq's jihadi battalion were able to capture American military man John Adam after killing a number of his comrades and capturing the rest," said the statement, signed by the "Mujahedin Brigades."
The claim, carried on the website ansarnet.ws, could not be verified. A US military spokesman in Baghdad, Marine Sergeant Salju K Thomas, said he had no information on the claim but "we are currently looking into it".
A group using the same name last month claimed responsibility for the kidnapping of a Brazilian engineer. More than 120 foreigners have been kidnapped in Iraq in the past year.Reuse content