A human rights group says the United States needs to need act faster and more transparently in its investigation into the abuse of prisoners held by the U.S. military in Afghanistan.

The U.S. advocacy group Human Rights Watch says the military needs to make public its investigations into alleged abuses.

In an open letter to Defense Secretary Donald Rumsfeld Monday, the group cited the deaths of six prisoners while in U.S. custody in Afghanistan, some of which have only just come to light. The Department of Defense later revealed that the actual number of deaths under investigation is eight.

Human Rights Watch says some of the cases date back more than two years, and says only two people have been charged in connection with the deaths.

In the letter to Secretary Rumsfeld, the group's executive director, Brad Adams, says the military has acted too slowly and with too much secrecy in its investigation.

Mr. Adams also says the probes into allegations of U.S. guards torturing or otherwise mistreating prisoners have only come in response to media attention. He specifically makes reference to an internal Pentagon report on the treatment of U.S. detainees in Afghanistan, calling for the document to be made public.

The U.S. military spokesman in Afghanistan, Major Mark McCann, said Monday that he could not comment on the classified review of detention operations. "At the current time, the report is still going through a review process, and we have not been given authorization to release that report," he said.

The U.S. military has, however, promised to make the report public eventually.

The investigation was ordered earlier this year by the senior U.S. commander, Lieutenant General David Barno, in response to allegations of mistreatment of Afghan detainees. According to Human Rights Watch, it has been almost six months since the report was completed.

Javid Ludden, spokesman for Afghan President Hamid Karzai, said Tuesday that the government is concerned about allegations of mistreatment, but would reserve judgment until more facts of the cases become known.

"The coalition forces are here, fighting alongside our forces against the enemies of Afghanistan, against terrorism," said Mr. Ludden. "But that does not mean that they should not follow certain principles."

Mr. Ludden says he believes the U.S. military takes such issues seriously. He adds that the Afghan people need to have confidence that all foreign troops in Afghanistan are acting in the country's best interest.