Between 50 and 100 detainees held by the US at Guantanamo Bay cannot be released or put on trial, US Defence Secretary Robert Gates has said.
The fate of those detainees "is still open", Mr Gates told members of the Senate Appropriations Committee.
The detention centre was set up in January 2002 for those captured in the US "war on terror".
Some 245 men are still held. In January President Barack Obama signed an order to close the prison within a year.
The US Justice Department is trying to determine which detainees will be taken by other countries or placed on trial.
"The question is what do we do with the 50 to 100 - probably in that ballpark - who we cannot release and cannot try," Mr Gates told the committee.
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"I think that question is still open."
When pressed on where the remaining inmates would eventually be held, Mr Gates declined to answer.
Mr Gates confirmed that although many of the detainees who are to be released without charge would be sent to other countries, some of the 17 Chinese ethnic Uighurs are likely to be allowed to settle in the US.
"What I have heard people talking about is our taking some of the Uighurs, probably not all," he said.
And he argued that if the US decided to take in some of the freed detainees, it would be easier to persuade other countries to accept them.