Parents of a US contractor executed by Islamic militants in Iraq are blaming the Bush administration for his death, saying that US authorities had contributed to the tragedy by unlawfully detaining their son for nearly two weeks before his disappearance.

Michael Berg claimed yesterday that his son, Nick, was held by US authorities without access to a telephone or lawyer because of suspicions that he was an insurgent or terrorist. By the time he was finally released in early April, according to Mr Berg, the security situation had so deteriorated that it was difficult for him to make his way home.

"That's really what cost my son his life, the fact that the United States government saw fit to keep him in custody for 13 days without any of his due process or civil rights," Mr Berg said.

Nick Berg, a small business owner from Pennsylvania, went to Iraq in December to help rebuild the country's infrastructure. Instead, he became the latest western contractor to have died a grisly death in that pursuit.

His headless body was found on a roadside by US soldiers yesterday. A video from an al-Qaeda-affiliated website depicted his decapitation by a gang of men who claimed to be taking revenge for the abuses committed by US soldiers at the Abu Ghraib prison.

The execution drew outraged comments from US leaders. White House spokesman Scott McClellan said those responsible had "no regard for the lives of innocent men, women and children".

Rick Santorum, a Republican senator from Mr Berg's home state, called it "an outrage to the civilised world", and said: "If anyone wants to know what we're fighting and why we're fighting this war on terrorism, this is a very good example of it."

The family's accusation comes as the Bush administration struggles to fend off allegations that it has denied the civil rights of thousands of mostly Arab detainees it is holding in military prisons around the world on suspicion of terrorism.

Mr Berg was apprehended by Iraqi officials near the northern city of Mosul on March 24, according to his father, who claims that he was then interrogated by FBI agents.

Mr Berg was released on April 6, a day after his parents filed a lawsuit in federal court naming defence secretary Donald Rumsfeld as a defendant, and claiming that their son was being unlawfully held. They last heard from him on April 9.

The FBI said it was premature to comment on the case. An official familiar with the case acknowledged that FBI agents had interviewed Mr Berg, but said he was in Iraqi - not American - custody. The official was unable to clarify the legal difference between the two, given the US occupation.

But Mr Berg's father disputed this, saying it was "the FBI", when asked in a radio interview who had detained his son.

"We named Rumsfeld as the responsible person, and I still hold him responsible," Mr Berg said. "It's the whole Patriot Act. It's the whole feeling of this country that rights don't matter any more because there are terrorists about," he added. Additional reporting by Edward Alden