The US army has admitted that eight detainees have died in its custody in Afghanistan - two more than it had previously acknowledged.
Donald Rumsfeld was told of claims of abuse and arbitrary detention
The army's admission came after the campaign group Human Rights Watch said it knew of three new incidents.
The group sent an open letter to US Defence Secretary Donald Rumsfeld, saying the US has failed to investigate or punish abuses by its soldiers.
A US official said at least three deaths were still being investigated.
"It's time for the United States to come clean about crimes committed by US forces in Afghanistan," said Brad Adams, Asia Division director for Human Rights Watch (HRW).
A failure to prosecute soldiers for incidents in Afghanistan had led to "a culture of impunity" which may have spread to Iraq, he said.
There was fury earlier this year when photographs emerged of Iraqi prisoners being physically and sexually abused by American prison guards.
The latest revelations are a further embarrassment for the US military and will only intensify scrutiny of the way it treats its prisoners, says the BBC's Ian Pannell in Washington.
The US military said eight deaths in Afghanistan had been documented.
It released only basic details of the cases, which included:
HRW says the documents only came to light after a request by another US campaign group under the Freedom of Information Act.
HRW says it is aware of "only a handful of criminal investigations" into the cases and into many claims of torture by detainees.
It says it knows of only two US personnel being charged with any crime.
There has been criticism of procedures at detention centres
The HRW letter also claims that the US-run detention system in Afghanistan operates entirely outside the rule of law, and that commanders have repeatedly refused requests for visits by the Afghan Independent Human Rights Commission.
HRW also urged the US military to publish a delayed internal report on its Afghan detention centres.
The US authorities promised to release parts of the report several months ago, but a spokesman said on Monday that no date had been set for publishing the report because its findings were still being reviewed in Washington - but that no abuses had been uncovered.