A video of an American being beheaded by his captors in Iraq was posted on a Web site linked to the Al Qaeda on Tuesday, according to CNN.

The United States refused to exchange him for prisoners in Baghdad's Abu Ghraib prison, the captors said.

The video is titled 'Abu Musab al-Zarqawi shown slaughtering an American'.

Shown on the Muntada al-Ansar site, it begins with the victim sitting on the floor with five masked men behind him.

The man, wearing an orange jumpsuit, identifies himself as Nicholas Berg, 26, of Pennsylvania.

The Central Intelligence Agency is said to be checking to see whether al-Zarqawi, an Al Qaeda operative accused of masterminding attacks against US forces in Iraq, was actually in the video.

The captors also issued a statement to the US President George W Bush: "The worst is coming and, god willing, the tough days are still to come. You and your soldiers will regret the day that you touched the ground of Iraq."

The one standing directly behind Berg reads a statement identifying himself, and then the American is pushed to the floor.

Berg is heard screaming as his throat is cut. One of the captors then holds up his severed head.

"For the mothers and wives of American soldiers, we tell you that we offered the US administration to exchange this hostage for some of the detainees in Abu Ghraib and they refused," the hooded man standing behind the American said just before the killing.

"Coffins will be arriving to you one after the other, slaughtered just like this."

CNN quoted White House spokesman Scott McClellan as saying, "This shows the true nature of the enemies of freedom. They have no regard for the lives of innocent men, women and children."