A video found on an Islamist website shows what appears to be the execution of an American hostage in Iraq.

The footage shows a young man identifying himself and saying he is from San Francisco, California. He is dressed in a plain beige T-shirt and sits nervously in a chair, as he warns that other Americans will be killed if U.S. forces do not leave Iraq.

The tape then shows him being beheaded.

No militants are seen on the video, unlike previous tapes of hostage killings.

The video has been linked to Abu Musab al-Zarqawi, a Jordanian-born suspected operative of the al-Qaida terrorist network, whose group, Tawhid and Jihad, has claimed responsibility for deadly attacks across Iraq and the beheadings of a U.S. businessman Nicholas Berg, a South Korean translator, and a Bulgarian truck driver. U.S. officials have not commented about the tape.

Dozens of hostages from more than 20 countries have been kidnapped in Iraq in the last four months. Most have been released, but at lest 20 are still being held.