The US embassy in Kabul said on Wednesday an Afghan police officer, reportedly held by US-led forces in the city of Gardez and the US base at Bagram in 2003, said he had been stripped naked, photographed, kicked and subjected to "sexual taunting".
The allegation will be of major concern to the 20,000-strong US-led force in Afghanistan, which until now has not faced the same level of resistance its troops have in Iraq since it helped topple the Taliban regime late in 2001.
"Yesterday afternoon, coalition leaders were notified of an allegation of detainee abuse," said US military spokesman Lieutenant Colonel Tucker Mansager, apparently referring to the police officer's complaint.
"Upon notification, coalition forces immediately launched an investigation into this matter. The investigation continues."
Graphic photographs of US soldiers abusing naked Iraqi prisoners have been shown across the globe, incensing the Arab world and damaging US credibility.
The US military is under pressure to allow the Afghan Independent Human Rights Commission access to suspected members of al-Qaida and the Taliban held at centres including at its main Afghan base at Bagram, just north of Kabul.
Mansager said the issue of wider access was being considered, but added: "The coalition believes that the International Committee of the Red Cross rightfully and properly represents the interests of persons placed under control in a proper manner.
"As they have in the past, they will continue to have access to our Bagram facility that they visit on a regular basis."
It is not known how many prisoners are being held at Bagram or who they are. An investigation into the deaths of two prisoners while in US detention in December, 2002, has yet to be completed.
The Afghan rights body says it has received complaints from more than two dozen detainees released from US custody about their treatment.