The US military says it is examining alleged cases of abuse of prisoners at detention centres in Iraq.
The coalition is holding at least 10,000 prisoners in Iraq
A statement said US commander Ricardo Sanchez had ordered the investigation after abuse was reported at an unnamed coalition facility.
The allegations are said to be serious and refer to more than one incident.
US-led forces are holding up to 10,000 prisoners-of-war in Iraq, and have faced criticism on the issue from tribal and community leaders.
International human rights groups have alleged that thousands of detainees are still being held without charge in often overcrowded and unsanitary conditions.
The US is in the process of releasing around 500 prisoners under an amnesty programme.
The initial statement gave no details about the scope of the investigation.
But military officials are now saying that abuse occurred on more than one occasion and had been reported from within the military command.
In other words, the allegations came from soldiers.
BBC Pentagon corresopndent Nick Childs says the fact that the investigation has been initiated by the central US military command in Iraq rather than an individual unit is unusual.
It suggests that senior commanders are taking the issue very seriously.
General Sanchez's statement said the coalition was "committed to treating all persons under its control with dignity, respect and humanity", adding that the investigation would be carried out in a thorough and professional manner.
The investigation was in its early stages so no specific information on the abuses could be released, it said.
Three soldiers were discharged from the US army for mistreating Iraqi prisoners of war at the Camp Bucca detention camp in southern Iraq last May.
An internal inquiry found soldiers had thrown prisoners down and kicked them in the head, groin and abdomen.