The White House this afternoon condemned the beheading of American Nick Berg in Iraq and scoffed at the reason the killers gave for the death.
A Web site video yesterday showed a masked man cutting off the head of Berg and said al Qaeda's leader in Iraq had personally carried out the killing in revenge for American abuses of Iraqi prisoners.
"I think the brutal, barbaric action of these terrorists shows their true nature. There simply is no justification for the deliberate, brutal killing of an innocent civilian," said White House spokesman Mr Scott McClellan.
As for the excuse the killers gave - revenge for prison abuses - Mr McClellan said: "The terrorists are going to seek any excuse and try to change their excuses to try to justify murder, destruction and chaos. They have no regard for innocent life. But we will not be deterred by the terrorists. We will defeat the terrorists and a free and peaceful Iraq will emerge."
President George W Bush did not see the Web video of the death but was briefed about it, the spokesman said.
"Nicholas Berg wanted to help build a free Iraq for the Iraqi people. He was an innocent civilian seeking to help," McClellan said.
The US administration has vowed to catch the group who beheaded an American civilian in Iraq.
The execution was carried out by a man who claimed to be Al Qaeda's leader in Iraq .
The leader said there would be more killings in revenge for the "Satanic degradation" of Iraqi prisoners by US soldiers, an Islamist Web site said.
A poor quality videotape on the site showed a man dressed in orange overalls sitting bound on a white plastic chair in a bare room, then knelt on the floor with five masked men behind him.
"My name is Nick Berg, my father's name is Michael... I have a brother and sister, David and Sarah," said the bound man, adding he was from Philadelphia.
One of the masked men read a statement urging Muslims to seek revenge after pictures were published of Iraqi prisoners being abused by US troops at Abu Ghraib prison near Baghdad.
"Nation of Islam, is there any excuse left to sit idly by? And how can free Muslims sleep soundly as they see Islam being slaughtered, honor bleeding, photographs of shame and reports of Satanic degradation of the people of Islam, men and women, in Abu Ghraib prison?" the statement said.
The masked men then pushed the 26-year-old American to the floor and shouted "God is greatest" above his screams as one of them sawed his head off with a large knife then held it aloft for the camera.
The ritual killing resembled the murder of US reporter Daniel Pearl, beheaded by Islamist militants in Pakistan, and Mr Berg's orange overalls were reminiscent of those worn by al Qaeda suspects held by US troops at Guantanamo Bay in Cuba.
The Web site said Abu Musab al-Zarqawi, a top ally of al Qaeda leader Osama bin Laden, was the man who cut off Berg's head. The statement in the video was signed off with Zarqawi's name and dated May 11th.
Jordanian-born Zarqawi, 37, has raised his profile and status as al Qaeda's most active operational leader with a series of suicide bombs and attacks on US troops in Iraq.
A body found in Baghdad over the weekend was identified on yesterday as Mr Berg before the videotape was released.
This shows the true nature of the enemies of freedom," White House spokesman Scott McClellan told reporters. "They have no regard for the lives of innocent men, women and children. We will pursue those responsible and bring them to justice."
"The Berg family is devastated by this loss. They want to extend their sympathy to other families who have also suffered," said Mr Bruce Hauser, their neighbour in the Philadelphia suburb of West Chester.
Mr Berg's murder brings to three the number of known deaths of civilians who have been kidnapped or seized in the last several months.
About 50 people have been kidnapped or gone missing, with several known to be still held by militants.