BAGHDAD, Iraq — A video posted Tuesday on an al-Qaida-linked Web site showed the beheading of an American civilian in Iraq and said the execution was carried out to avenge abuses of Iraqi prisoners at Abu Ghraib prison.

In a grisly gesture, the executioners held up the man's head for the camera.

The American identified himself on the video as Nick Berg, a 26-year-old Philadelphia native. His body was found near a highway overpass in Baghdad on Saturday, the same day he was beheaded, a U.S. official said.

The video bore the title "Abu Musab al-Zarqawi shown slaughtering an American." It was unclear whether al-Zarqawi — an associate of Osama bin Laden believed behind the wave of suicide bombings in Iraq — was shown in the video or simply ordered the execution. Al-Zarqawi also is sought in the assassination of a U.S. diplomat in Jordan in 2002.

The Bush administration said those who beheaded Berg would be hunted down and brought to justice.

"Our thoughts and prayers are with his family," White House Press Secretary Scott McClellan said. "It shows the true nature of the enemies of freedom. They have no regard for the lives of innocent men, women and children."

Berg was a small-business owner who went to Iraq independent of any organization to help rebuild communication antennas, his family said Tuesday. Friends and family said he was a "free spirit" who wanted to help others — working in Ghana, in one example — and that his going to Iraq fit with that ideology. They said he supported the Iraqi war and the Bush administration.

U.S. officials had feared the shocking photographs of U.S. soldiers abusing and humiliating Iraqis at Abu Ghraib prison west of Baghdad would endanger the lives of American troops and civilians.

Also, Berg's killing happened amid a climate of intense anti-Western sentiment, which flared in Iraq after last month's crackdown on Shiite extremists and the three-week Marine siege of Fallujah west of Baghdad. Anger at the United States swelled with the publication of the Abu Ghraib photographs, which continue to stir rage throughout the Arab world.

In the video, five men wearing head scarves and black ski masks stand over a bound man in an orange jumpsuit similar to prison uniforms.

"My name is Nick Berg. My father's name is Michael. My mother's name is Suzanne," the man, seated in a chair, says. "I have a brother and sister, David and Sara. I live in . . . Philadelphia."