Islamic militants are threatening to behead a man they say is a US marine they took hostage in Iraq.
The US would not confirm the hostage's identity
Footage aired on the al-Jazeera television channel showed a blindfolded man dressed in camouflage and holding a US marine ID card.
The card said the man's name was Wassef Ali Hassoun and he was on active duty.
The US military would not confirm that Mr Hassoun had been taken hostage, but his family in Utah said he was the man pictured and pleaded for his release.
Islamist groups have falsely used military ID cards in the past to claim that they have taken US soldiers hostage.
The missing soldier's ID card was among documents displayed
Al-Jazeera said the man was of Pakistani origin, whereas the US military says the missing soldier, a member of the 1st Marine Expeditionary Force, is of Lebanese descent.
"Naval Criminal Investigative Services cannot confirm that Cpl Wassef Ali Hassoun has been taken hostage," the US military said in a statement.
But a friend of his Muslim family in the US read out a statement on their behalf, saying: "In the name of Allah, the merciful, the compassionate, we accept destiny with its good and its bad.
"We pray and we plead for his safe release and we ask all people of the world to join us in our prayers. May God bless us all."
Al-Jazeera said the group called itself Islamic Response, the security wing of the 1920 Revolution Brigades, referring to the uprising against the British after World War I.
The group said they would will kill the man if Iraqi prisoners are not released.
The militants claim they were able to abduct the man after infiltrating a US military camp and then luring him outside.
String of abductions
Earlier al-Arabiya TV showed pictures of a Pakistani driver taken hostage by militants.
The tape showed Amjad Hafeez holding an ID card from US contractor Kellogg, Brown and Root, which provides support to the US military.
A separate group claims to have abducted a Pakistani driver
"This man was taken after an attack on a US base in Balad," one of the three masked gunmen on the tape said.
"You must release our prisoners held near the US base in Balad, in Dujail, in Yethrib, 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," he added.
On the video the prisoner urged Pakistan's President Pervez Musharraf to close Pakistan's embassy in Iraq.
Pakistan said it would try to negotiate his release, but added that there was no way it would bow to the militants' demands.
On Saturday, in another video broadcast by al-Jazeera, suspected supporters of al-Qaeda leader Abu Musab al-Zarqawi said they had taken three Turkish men captive in Iraq.
They threatened to behead the men unless all Turkish firms withdrew from Iraq within 72 hours.
Turkey has rejected the demand.
Zarqawi's movement has already claimed responsibility for the beheading of two other hostages captured in Iraq - US man Nick Berg and South Korean Kim Sun-il.