This makes me sick to my stomach.
BAGHDAD, Iraq (CNN) -- An al Qaeda-linked Web site showed video Tuesday of a man who identified himself as an American and then was beheaded.
His captors said the United States refused to exchange him for prisoners in the Abu Ghraib prison.
"For the mothers and wives of American soldiers, we tell you that we offered the U.S. administration to exchange this hostage for some of the detainees in Abu Ghraib and they refused," says a hooded man standing behind the American.
"So we tell you that the dignity of the Muslim men and women in Abu Ghraib and others is not redeemed except by blood and souls. You will not receive anything from us but coffins after coffins, slaughtered in this way."
The video was discovered as a U.S. Senate committee heard testimony on the U.S. military investigation into abuse of Iraqi prisoners at Abu Ghraib. (Full story)
At the beginning of the tape, the victim describes himself as Nicholas Berg from Pennsylvania.
"My name is Nic Berg. My father's name is Michael. My mother's name is Suzanne. I have a brother and a sister -- David and Sarah," says the man.
"I live in West Chester, Pennsylvania, near Philadelphia."
He is then shown sitting in front of five hooded men. After the statement is read by one of the men, the victim is pushed to the floor and, amid his screams, his throat is cut. Finally, one of the captors holds up the man's severed head.
Earlier in the day, the State Department identified the body of an American found Monday in Baghdad as that of Nicholas Berg of Pennsylvania.
Berg was not a soldier or a civilian employee of the Pentagon, the State Department said.
A senior State Department official also said that the beheaded body of an American had been found, but he could not confirm it was Berg.
The Web site said the killing had been carried out by Abu Musab al-Zarqawi, the leader of an Islamist terrorist group that has claimed responsibility for numerous attacks on coalition forces in Iraq.
The voice on the tape could not be verified as that of al-Zarqawi.
Earlier Tuesday U.S. military officials said American soldiers have killed 13 Iraqi militia loyal to renegade Shiite Muslim cleric Muqtada al-Sadr and have detained 14 others in overnight fighting near Najaf.
Late Monday, members of al-Sadr's Mehdi Army attempted to ambush a U.S. convoy with small arms fire. No American forces were injured in the incident.
The U.S. military began a major offensive Sunday to retake control of parts of the city, reclaiming the governor's palace from al-Sadr's control.
Al-Sadr, who is wanted in connection with the killing of a rival cleric last year, launched an uprising against U.S. troops in April. Since then, coalition forces have cracked down on the cleric's strongholds while trying to avoid religious sites.
Al-Sadr is believed to be in Najaf, where his militia remains in control of much of the Shiite holy city, although he travels to nearby Kufa to deliver weekly sermons on Friday.