The US military, under intense scrutiny for its treatment of Islamic militant suspects, is looking into a new allegation of prisoner abuse in Afghanistan, a spokesman said on Saturday.
Major Jon Siepmann declined to give details but told reporters the Naval Criminal Investigation Service, which has responsibility for the US Marines as well as the Navy, was carrying out the investigation.
There are about 2,000 Marines on combat duty in Afghanistan.
"Because it is currently under investigation, we cannot provide any specific details regarding the allegation," Siepmann said. "(We) will take appropriate action based on the outcome of the investigation."
Revelations of ill-treatment of military prisoners in Iraq, Afghanistan and at the Guantanamo Bay US Naval base in Cuba have tarnished the reputation of US forces.
The military says it has investigated five deaths of prisoners in Afghanistan since August 2002.
News of the fresh investigation comes just weeks after a CIA contractor was arrested on charges of beating a detainee who died in 2003, the first brought in connection with prisoner abuse in Afghanistan.
After news broke concerning treatment of prisoners in Iraq, the US military announced a sweeping review of its prison system in Afghanistan, a report on which is expected this month. The review was launched after allegations of abuse by former detainees, including a former policeman who said he was beaten and sexually abused.
The US Justice Department is pressing the CIA to publicly reveal the specific interrogation methods authorised by the Bush administration for a handful of senior al Qaeda captives, the Washington Post reported on Saturday.
The interrogation methods have been classified since they were first used in questioning al Qaeda suspects picked up in Afghanistan and elsewhere after the September 11 attacks.
The US-based rights group Human Rights Watch has called abuse of detainees in Afghanistan "systemic", and criticised the US decision not to grant suspected militants prisoner-of-war status that would give them rights under the Geneva Conventions.
This month the military allowed the International Committee of the Red Cross - which has a global role in checking on conditions for war detainees - access to a detention facility in Kandahar that had been thought to be only a transit post.
Siepmann said he understood "there have been generally positive reports of that visit thus far". News of the latest investigation came as US-led forces have been conducting operations in southern and central Afghanistan aimed at increasing security for elections later this year.
Siepmann said about 12 militants has been killed and a similar number captured in operations in the past week.
In a previous drive launched in late May, the US military said it killed more than 80 guerrillas and detained about 90 suspects, but militants attacks have continued.