WASHINGTON (Reuters) - Eight detainees have died in American military custody in Afghanistan, more than previously reported, the Pentagon said on Monday, while a human rights group assailed a U.S. "culture of impunity" on prisoner abuse.

New York-based Human Rights Watch complained in a letter to Defense Secretary Donald Rumsfeld that in most instances, the Pentagon has launched criminal probes into detainee deaths in Afghanistan only after cases get media attention, and that these probes have proceeded slowly and in excessive secrecy.

"The government's failure to hold its personnel accountable for serious abuses has spawned a culture of impunity among some personnel. And as you know, some of the personnel involved in earlier abuses in Afghanistan have now been implicated in later abuses in Iraq," the letter stated.

"It's time for the United States to come clean about crimes committed by U.S. forces in Afghanistan," added Brad Adams, Asia Division director for Human Rights Watch.

The group's letter referred to six detainees who have died in U.S. custody in Afghanistan, where American forces have been involved in military operations since October 2001.

Chris Grey, a spokesman for the Army Criminal Investigation Command, said investigators actually have looked into the deaths of eight detainees in U.S. military custody. The earliest death listed by Grey was in August 2002 and the most recent was in September.

"We investigate all deaths of detainees regardless of the circumstances," said Air Force Lt. Col. John Skinner, a Pentagon spokesman. "Deaths may have occurred as a result of injuries sustained prior to detention by U.S. forces, natural causes such as pre-existing medical conditions or other reasons."

'NOT TOLERATED'

"Our standard has always been and remains to treat detainees humanely. Anything less is not tolerated," Skinner said.

Human rights groups have sharply criticized the United States for prisoner abuse at the hands of its forces in Iraq and Afghanistan. Photographs of American soldiers beating, tormenting and sexually humiliating Iraqi prisoners at Abu Ghraib jail surfaced in April, drawing worldwide condemnation.

In a March report, Human Rights Watch referred to cases of U.S. personnel in Afghanistan arbitrarily detaining civilians, using excessive force while arresting noncombatants, and mistreating detainees. The report said prisoners held at U.S. bases in 2002 and 2003 had described being beaten severely by guards and interrogators, deprived of sleep for extended periods, and intentionally exposed to extreme cold.

The Human Rights Watch letter lamented that the U.S. government has conducted only a handful of criminal investigations into the Afghan detainee deaths, and has charged only two people with crimes.

The Army said in October its criminal investigators had recommended possible charges ranging from manslaughter to conspiracy against 28 soldiers in the deaths of two prisoners at Bagram Air Base in Afghanistan in 2002.

Sgt. James Boland, an Army reservist in a military police unit, was earlier charged with assault, maltreatment and dereliction of duty in the deaths, officials said.

The two prisoners died on Dec. 4 and Dec. 10, 2002, after blunt force injuries, according to the Army. Military medical examiners classified their deaths as homicides.

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