US Defence Secretary Donald Rumsfeld has described as "reprehensible" Amnesty International's description of the Guantanamo prison as a gulag.
The group has said Mr Rumsfeld should be held accountable for torture.
The dispute between the Pentagon chief and the human rights group came a week after Amnesty compared the prison for foreign terrorism suspects at the US naval base at Guantanamo Bay, Cuba, to the vast, brutal Soviet system of forced labour camps in which millions of prisoners died.
"No force in the world has done more to liberate people that they have never met than the men and women of the United States military. Indeed, that's why the recent allegation that the US military is running a gulag at Guantanamo Bay is so reprehensible," Mr Rumsfeld told a Pentagon briefing.
The United States holds about 520 men at Guantanamo, where they are denied rights accorded under international law to prisoners of war. Many have been held without charge for more than three years.
Free societies welcome informed criticism, particularly on human rights, but "those who make such outlandish charges lose any claim to objectivity or seriousness," Mr Rumsfeld said.
Amnesty issued a statement noting that in the early weeks of the Iraq war, Mr Rumsfeld at least three times cited Amnesty's account of human rights violations by Iraq's then-president, Saddam Hussein.
"Twenty years ago, Amnesty International was criticising Saddam Hussein's human rights abuses at the same time Donald Rumsfeld was courting him," said William Schulz, executive director of Amnesty International USA, referring to
Mr Rumsfeld's December 1983 meeting in Baghdad with Saddam as a special presidential envoy.
"In 2003, Rumsfeld apparently trusted our credibility on violations by Iraq, but now that we are criticising the US he has lost his faith again," Mr Schulz said.
Current US policy is to imprison people without charge or trial at Guantanamo, Bagram Air Base in Afghanistan and other places where treatment does not meet international standards, Mr Schulz said.
"Donald Rumsfeld personally approved a December 2002 memorandum that permitted such unlawful interrogation techniques as stress positions, prolonged isolation, stripping, and the use of dogs at Guantanamo Bay, and he should be held accountable, as should all those responsible for torture, no matter how senior," Mr Schulz said.
President George W. Bush called Amnesty's gulag comparison "an absurd allegation," adding: "The United States is a country that promotes freedom around the world."