The US State Department said it did not want to release detainees at Guantanamo Bay only for them to commit acts of terrorism.
US State Department spokesman Sean McCormack insisted that "at some time in the future we would like nothing better" than to close the camp.
He said dangerous people were still being detained there, and the administration did not want to release them only to see them resume attacks on US and other personnel.
A Pentagon spokesman said: "The dangerous detainees at Guantanamo include terrorist trainers, bomb makers and would-be suicide bombers, many who have vowed to return to the fight."
US officials were responding to a call from the British Attorney General, Lord Goldsmith, to close the controversial detention camp at the Cuban military base.
In a speech to a terrorism conference organised by the Royal United Services Institute, Lord Goldsmith said the United States's long tradition as a "beacon of freedom, liberty and justice" deserved to see Camp Delta consigned to history.
"The existence of Guantanamo Bay remains unacceptable," said Lord Goldsmith. "It is time, in my view, that it should close."
Lord Goldsmith is the minister responsible for advising the British government on the law, and was involved in negotiations with the US over Britons in Guantanamo.
Prime Minister Tony Blair has never called for Guantanamo's closure but has described the base as an "anomaly".
Foreign Office minister Kim Howells said in March that the camp should close if it was thought to be undermining the cause of democracy.
Also in March, US Attorney General Alberto Gonzales insisted during a visit to London that the camp was lawful and necessary.