US military spokesman Tucker Mansager gave few details on Saturday of the latest allegations.
They follow complaints earlier this week from an Afghan police officer who said he was beaten and sexually assaulted during 40 days in custody last summer.
"On Thursday, coalition leaders were notified of another allegation of detainee abuse. Upon notification coalition forces launched an immediate investigation," Mansager told a news conference in the capital Kabul.
He said the detainee was arrested last year and had since been released. He did not divulge who provided the information that prompted the new probe.
Rights groups have long complained of what they say are consistent allegations of abuse in American holding facilities across the country since the fall of the Taliban in late 2001, where hundreds of alleged Taliban and al-Qaida fighters have been held without legal access or charge.
The US military classifies them as "unlawful combatants," a designation that strips them of the rights afforded by the Geneva Conventions, and some have been transferred to Guantanamo Bay, Cuba.
The scandal over abuses at Abu Ghraib prison in Iraq appear to have spurred the military into a quick response into the recent reports of mistreatment in Afghanistan.
Some Afghan prisoners were
transferred to Guantanamo
On Monday, the US military opened a criminal investigation into complaints of mistreatment by the Afghan police officer, Sayyid Nabi Siddiqui, after his graphic account of his detention was published in the media.
Siddiqi told The Associated Press he was punched, stripped naked and had objects inserted into his anus in three US holding facilities before being released without charge.
Human Rights Watch on Thursday said his case fit a pattern and called for US holding facilities in the country there to be opened to outside scrutiny. It claimed prisoners in US custody suffered "systemic" mistreatment.
The US-based rights group also urged the United States to finally clear up the deaths of three Afghans in custody since late 2002.
Military autopsies have ruled two deaths of prisoners at the main coalition base at Bagram in December 2002 were homicides caused by blunt force injuries, but the military has yet to announce the findings into its investigation, which it says is ongoing.
"We have a duty to ensure that our troops are sent into battle only as a last resort"
US Democratic presidential candidate
Very little is also known about the death of a third prisoner in eastern Kunar province in June 2003. A US intelligence official said the CIA inspector-general was investigating that death because it involved an independent contractor working for the agency.
The probe comes as President Bush's political rivals exploit the issue to pile on the pressure.
US Democratic presidential candidate John Kerry said on Saturday the United States should never fight wars of choice - only those of necessity - in an implicit repudiation of President George Bush's military campaign in Iraq.
The remarks came as new opinion polls indicated Bush's political fortunes continued to sink in the face of raging violence in Iraq and a prisoner abuse scandal that has dealt a blow to US strategy in the region.
The CNN-Time survey found the number of people with doubts and reservations" about Bush's ability to lead the country rose from 55% to 59%, while those who believe the president can be trusted as a leader dropped from 44% to 39% since early February.
Bush approval rating
Bush's overall job approval rating fell from 49% to 46% since early April, positioning Kerry to win 51% to 46% if an election were held today.
"We have a duty to ensure that our troops are sent into battle only as a last resort," said the Massachusetts senator, delivering a Democratic weekly radio address.
Kerry says US troops should have
'a clear sense of mission'
"This nation should never go to war because it wants to, but only because it has to," he continued.
Kerry said he believed US troops should have "a clear sense of mission" before they are sent into harm's way.
"We have a duty to look ahead so that once victory on the battlefield is won, we have a plan to win the peace," he said.
"We have a duty to build and lead alliances so that our troops and our country will not have to bear almost alone the burdens of defending freedom and defeating the great dangers that now threaten our national safety and global security," the senator stressed while avoiding mentioning Bush by name.