A video posted on a website used by Islamic militants purported to show the beheading of an American hostage today but the man shown in the video was quoted as saying it was a hoax.
Mr Benjamin Vanderford of San Francisco, California, told the Associated Press he had faked the video at a friend's house.
The video appeared in the web format used by al Qaeda ally Abu Musab al-Zarqawi and was introduced by a headline that said it showed Zarqawi killing an American.
The American then identifies himself as Benjamin Vanderford and urges US forces to leave Iraq.
"If we don't, everyone is gonna be killed in this way ... I have been offered for exchange for prisoners here in Iraq," the terrified-looking man said, rocking back and forth in his chair, his hands tied behind his back.
The video then showed a hand with a large knife apparently slicing through the neck of a limp body.
Mr Vanderford told the AP he staged the beheading using fake blood and began distributing the videotape on the Internet months ago. "It was part of a stunt, but no one noticed up until now," he said.
Mr Vanderford's website identifies him as a 22-year-old independent candidate for district supervisor in San Francisco. He is also a musician and video-game programmer.
His political manifesto on the Web site addresses local issues such as tenants' rights, the homeless and government transparency, with no mention of Iraq.
The video on the website looked like previous tapes released by Zarqawi's Tawhid and Jihad Group, but did not show the group's flag, or masked militants. The American was wearing trousers and a T-shirt, not an orange jumpsuit as worn by previous hostages of Zarqawi's group.
But the statement, signed by the Tawhid and Jihad Group, bore the same logo that had appeared in previous statements including most recently Saturday's threat to kill Iraqi Prime Minister Iyad Allawi.
Zarqawi's group has claimed responsibility for several suicide bombings and other attacks on Iraqi and U.S. officials in recent months. It has also previously killed an American, a South Korean and a Bulgarian hostage in Iraq.
Reports and pictures of US soldiers abusing Iraqi prisoners have sparked outrage in the Arab world and the international community, and have hampered US efforts to stabilise the country amid a bloody insurgency.
Scores of hostages from two dozen countries have been seized in the last four months. Most have been freed but at least 10 have been killed, and at least 20 are still being held in Iraq.