The US military had closed an investigation into an alleged rape of a female detainee for lack of evidence but was still looking into another alleged rape case, a Pentagon spokesman said.
Pentagon spokesman Bryan Whitman said: "One of those cases has been thoroughly investigated and was closed, and there was insufficient evidence. The second one is an ongoing investigation."
Allegations of sexual misconduct by US military personnel surfaced last year during the Abu Ghraib prison scandal, but these are the first known cases of soldiers being accused of raping female detainees.
Whitman said he did not know when or where the rapes were alleged to have occurred.
Under US control
"You've got prisoners, detainees under the control of the United States. Some allegations were made.
Rape charges were only disclosed
after senators asked Rumsfeld
"The allegations are being investigated or have been thoroughly investigated, and to date there is no substantiation of any allegation that a US service member has raped an Iraqi female prisoner," he said.
The Pentagon disclosed the investigations only after senators last week asked US Defence Secretary Donald Rumsfeld and General Richard Myers, the chairman of the Joint Chiefs of Staff, whether any US personnel were reported to have raped Muslim women.
Rumsfeld and Myers said they did not know. Rumsfeld cited the voluminous reports on investigations triggered by the Abu Ghraib scandal.
"And whether buried in there, there's that particular allegation that you cited, I'll be happy to try to find out for you," Rumsfeld said.
Fighters in Iraq have cited abuse of Iraqi women detainees by US forces as justification for their attacks.
Sexual abuse of detainees first came to light last year with the discovery of digital images of naked detainees at Abu Ghraib being humiliated by US forces, prompting an investigation by Major-General Antonio Taguba.
Three soldiers were punished for
making a detainee open her blouse
In one case stemming from Abu Ghraib, three soldiers received non-judicial punishment for taking a female detainee to an isolated part of the prison and making her open her blouse.
Non-judicial punishment is meted out by an accused soldier's commander without a court martial. Punishment includes forfeiture of pay, reprimand and detention. It does not constitute a criminal conviction.
Specialist Charles Graner, who was sentenced last month to 10 years in prison for mistreating prisoners at Abu Ghraib, had a female detainee lift her blouse and photographed her in another incident disclosed at his court martial.
The Justice Department is also investigating an allegation that a civilian security contractor at the prison raped a young male detainee.