A video posted on a website linked to Iraqi militants apparently shows the beheading of an American civilian, and threatens to kill two more hostages taken last week.

Although the authenticity of the nine-minute video has not been confirmed, it shows a masked man sawing American construction contractor Eugene Armstrong's head off with a knife.

The masked man is claimed to be top terror suspect Abu Musab al-Zarqawi. Audio experts said the voice resembled that of Zarqawi.

In a statement attributed to Zarqawi's al-Qaeda-linked group, Tawhid and Jihad, a voice can be heard claiming they have "beheaded the first American."

Speaking on condition of anonymity, a U.S. official told Reuters that Armstrong's body "has been recovered and it's been identified."

Armstrong, fellow American Jack Hensley and Briton Kenneth Bigley were taken hostage in Baghdad last Thursday.

The hostage-takers are demanding that Iraqi women detained in U.S.-run prisons be released; although the U.S. denies holding any women.

In the online statement Monday, the group promises that the two other captives face the same fate in the next 24 hours.

"The commander Abu Musab al-Zarqawi, God protect him, today slaughtered the first American hostage ... the group will carry out the slaughter of the others next ... We will provide you with film showing the slaughter soon," an Associated Press translation of the Arabic statement read.

The message was signed by Abu Maysarah al-Iraqi, a pseudonym used in the past to post messages on the website for the Tawhid and Jihad group.

More than 100 foreigners have been kidnapped in Iraq this year, and some have been executed.

Zarqawi's group has claimed responsibility for six hostage beheadings, and most of the bloodiest suicide bomb attacks in Iraq since the fall of Saddam Hussein.

Zarqawi is believed to be hiding out in Fallujah, a city 60 km. west of Baghdad. U.S. warplanes have been striking Fallujah almost daily, in order to attack the hideouts of Tawhid and Jihad.

Fallujah, part of an area known as the Sunni Triangle, remains outside the control of the U.S.-backed Iraqi interim government or the U.S. military.