Military officials said eight prisoner deaths in Afghanistan have been investigated since mid-2002, a higher number than previously reported.
Human Rights Watch said slow-paced investigations had 'spawned a culture of impunity' that may have fuelled prisoner abuse in Iraq.
"It's time for the United States to come clean about crimes committed by US forces in Afghanistan," Brad Adams, the group's Asia division director, said on Monday.
Failure to prosecute incidents in Afghanistan has allowed abusive interrogation techniques to spread to Iraq, Adams said. "The US government is dragging its feet on these investigations," he said.
A Pentagon spokesman, Lt. Col. John Skinner, said commanders go to "enormous lengths to investigate any credible allegations of detainee abuse". Many death investigations have determined that detainees died due to natural causes or because of injuries suffered before their capture, he said.
He added that the military has examined its entire system of keeping enemy prisoners.
"We've looked at detention operations from A to Z," he said.