The response follows a letter from Human Rights Watch which says it has discovered two more cases of prisoners dying in American custody in Afghanistan.

It says the cases include an apparent murder more than two years ago, and says slow-paced investigations have spawned a culture of impunity which may have fuelled prison abuse in Iraq.

In an open letter to US Secretary of Defence Donald Rumsfield, the New York-based rights group presented new evidence of an alleged murder of a detainee by four US military personnel in Afghanistan in 2002.

It also says a man picked up on September the 24th this year died the next day at an American base, but did not specify the cause of death.

Responding to the report the US Army acknowledged that eight prisoners have died in US military custody in Afghanistan, two more than previously disclosed.

The Pentagon released a list of the death cases investigated in Afghanistan after Human Rights Watch sent the letter to US Defence Secretary Donald Rumsfeld demanding that the United States “get serious about prosecuting people implicated in prisoner deaths and mistreatment.”

A spokesman for the US Army Criminal Investigation Command, Chris Grey said investigations in at least three of the deaths are still ongoing.

Human Rights Watch said along with the previously unreported cases a recently released internal Defence Department document states that an investigation was opened September 26, 2002 into an alleged murder of an Afghan detainee by four soldiers in or before September 2002.

The document indicated that the case was closed and a commander’s report of disciplinary or administrative action was issued, Human Rights Watch said.

“Yet we know of no courts martial which have taken place with respect to this ‘murder,'” it said.

In its letter to Mr Rumsfeld, Human Rights Watch charged that in most cases the Defence Department has launched criminal investigations only after particular abuses received media attention.

“These investigations have proceeded extremely slowly and in excessive secrecy,” it said. “An internal Pentagon investigation of detention operations in Afghanistan, conducted by Brig. Gen. Charles H. Jacoby, has been completed, but remains classified, unlike similar reports on abuses in Iraq.”