This comes as the Bush administration tried to contain growing outrage over the abuse of Iraqi detainees.
President George Bush was due to appear on two Arab television networks on Wednesday to say that he views the abuse as "shameful and unacceptable," said his spokesman, Scott McClellan.
Photographs showing Iraqi prisoners stripped of their clothes and being humiliated and abused have badly damaged US prestige and credibility, especially in the Arab world.
"The actions of the soldiers in those photographs are totally unacceptable and un-American," Defence Secretary Donald Rumsfeld said. "Any who engaged in such action let down their comrades who serve honourably each day and they let down their country."
Army officials said the military had investigated the deaths of 25 prisoners held by American occupation forces in Iraq and Afghanistan and determined that an army soldier and a CIA contractor each murdered a prisoner. Most of the deaths occurred in Iraq.
An army official said a soldier was convicted in the US military justice system of homicide for shooting a prisoner to death in September 2003 at a detention centre in Iraq.
The official, speaking on condition of anonymity, said a private contractor who worked for the CIA was found to have committed the other homicide against a prisoner.
US Secretary of State Colin Powell, at the United Nations for consultations on the Middle East, said only a "small number" of American troops had been involved in the abuse and
vowed wrongdoers would be quickly brought to justice.
Concerned about the fallout, Bush will appear on US-funded Arabic television channel Al-Hurra and the pan-Arabic television station Al-Arabiya to address the issue.
Bush will speak on Arab TV
channels in reaction to abuse
McClellan said Bush first learned of allegations of abuse sometime after the charges were elevated to top military officials in January. Pressed by reporters, McClellan said he could not narrow down the time frame any further.
Six US soldiers have been reprimanded and six others face criminal charges in connection with abuses at the Abu Ghuraib prison outside Baghdad, but Iraqi prisoners have complained of inhumane treatment by US troops at other centres as well.
The allegations first surfaced on 13 January but were only made public last week.
Several detention centres
The new US commander overseeing military-run prisons in Iraq, Major-General Geoffrey Miller, said there are an estimated 8000 detainees, about half of them at Abu Ghuraib.
The two other main prisons are at the southern port city of Umm Qasr and at the Baghdad International Airport, The Washington Post reported.
Miller said the US military runs 11 other detention facilities in Iraq where prisoners can be held for up to 14 days before a decision is made to release them or transfer them to one of the three main prisons, the newspaper said.
US military report: 'Abuses were
sadistic, blatant, criminal acts'
The images of abuse have dented the US argument that it invaded Iraq to bring democracy and human rights to a nation "brutalised" by former President Saddam Hussein.
Shocking excerpts from a report on the abuse completed on
3 March by Major General Antonio Taguba were likely to further stoke fury at home and abroad.
"Between October and December 2003, at the Abu Ghuraib Confinement Facility (BCCF), numerous incidents of sadistic, blatant, and wanton criminal abuses were inflicted on several detainees," the report said.
"This systemic and illegal abuse of detainees was intentionally perpetrated by several members of the military police guard force (372nd Military Police Company, 320th Military Police Battalion, 800th MP Brigade), in Tier (section) 1-A of the Abu Ghuraib Prison (BCCF)."
Taguba said several detainees had credibly described acts of abuse, including:
• Breaking chemical lights and pouring the phosphoric liquid on detainees
• Beating detainees with a broom handle and a chair
• Sodomising a detainee with a chemical light and perhaps a broom stick
• Arranging naked male detainees in a pile and then jumping on them
• Forcing detainees to remove their clothing and keeping them naked for several days at a time
• Forcing groups of male detainees to masturbate themselves while being photographed and videotaped
On Capitol Hill, angry Republicans and Democrats denounced the abuses. The Senate Intelligence Committee scheduled a closed hearing for Wednesday.
"The prisoner abuse is so disgusting, so degrading, that I think humanity has been hurt broadly," said Senate Majority Leader Bill Frist, a Tennessee Republican. He said it could undercut US efforts to bring democracy to Iraq.
Senator John McCain, an Arizona Republican and committee member who was a prisoner of war in Vietnam, joined many lawmakers in complaining that Rumsfeld and other top Pentagon officials failed to inform Congress of the situation.