Pentagon spokesman Major Michael Shavers told Britain's Financial Times newspaper on Monday the 25% figure was the opinion of a Guantanamo interrogator and not a Department of Defence estimate.
And with dwindling numbers of prisoners considered to have significant information, the newspaper reported that more detainees are expected to be released in the next couple of weeks.
In December 2004, the Pentagon proposed building a more permanent facility at Guantanamo called Camp Six to hold prisoners who are neither transferred nor released.
The proposed $25 million camp would have a capacity of 200 beds, more communal living space and a psychiatric wing, a defence official said.
"Right now it only exists on paper. It's not funded," said Shavers of the Camp Six plan.
"But it's only logical to take a look at the future of detention operations, especially as you consider the fact that the Camps One through Four are reaching end of their designed life."
"No decision has been made on who would be in such a camp," he said.
The detention centres at Guantanamo went into operation on 11 January 2002 as the first captives arrived hooded and shackled on military flights from Afghanistan and were put in cage-like cells at a facility called Camp X-Ray.
Camp X-Ray was replaced as the centre added five more camps to hold a population that at one point surpassed 700 prisoners, most but not all of whom were detained in Afghanistan.
Since then just over 200 prisoners have been transferred to their home countries to be held or released.
The US asserts the right to hold prisoners as "enemy combatants" for the duration of its "war on terrorism," and contends that they are not protected by the Geneva Conventions covering prisoners of war.
However, the US Supreme Court has ruled that prisoners can contest their detention in US federal courts.
The treatment of prisoners at Guantanamo has come under fire with disclosures most recently that FBI agents reported witnessing a variety of abuses there.