In a video broadcast by Aljazeera on Sunday, the Islamic Retaliation Movement/Armed Resistance Wing said US marine Hassoun Wassef Ali would be beheaded if detainees in US-led occupation prisons were not freed.
The group claims to have taken Ali captive after "infiltrating a US military base in Iraq".
The video received by Aljazeera shows a kneeling, blindfolded, moustached man in camouflage military garb. A hand holding a long sword is seen standing behind Ali.
US occupation forces acknowledged that a US marine had been reported missing.
US marine corporal Wassef Ali Hassoun has been missing in Iraq since last Monday, a spokesman for the US Marine Corps confirmed in Baghdad late on Sunday.
"Although we can't confirm that he has been taken hostage, we can confirm that that he has been absent from his unit since June 21," Major Douglas Powell said.
Hassoun was attached the the First Marine Expeditionary Force, and his unit was operating west of Falluja, Powell said.
Pakistani driver seized
Just hours earlier, an unidentified group of purported Iraqi fighters threatened to behead a Pakistani driver working for a US-occupation contractor within three days.
The captors also urged Pakistani President Pervez Musharraf to shut down his country's embassy in Iraq.
The second captive was named as Yusuf Amjad, an employee of US contractor Kellogg Brown and Root (KBR).
KBR is a subsidiary of the giant Halliburton company which was once headed by US Vice President Dick Cheney.
Video sent to Aljazeera included
proof of the marine's identity
Like other contractors for the US-led occupation, it has suffered mounting attacks on its staff.
"This man was taken after an attack on a US base in Balad," said one of the masked fighters on a tape obtained by Al-Arabiya.
"You must release our prisoners held near the US base in Balad, in Dujail, in Yathrib, in Samarra and near Abu Ghraib. You have three days from the date of this recording and after that we will behead him. We have warned you."