The admission was made in a report submitted to the UN Committee against Torture, said a member of the 10-person panel, speaking on condition of anonymity on Friday.
"They are no longer trying to duck this, and have respected their obligation to inform the UN," the committee member said.
"They will have to explain themselves. Nothing should be kept in the dark."
UN sources said it was the first time the world body had received such a frank statement on torture from US authorities.
The committee, which monitors respect for the Convention against Torture and Other Cruel, Inhuman or Degrading Treatment or Punishment, is gathering information from the US ahead of hearings in May 2006.
Signatories to the convention are expected to submit to scrutiny of their implementation of the 1984 convention and to provide information to the committee.
The document from Washington will not be formally made public until the hearings.
"They haven't avoided anything in their answers, whether concerning prisoners in Iraq, in Afghanistan or Guantanamo, and other accusations of mistreatment and of torture," the committee member said.
"They said it was a question of isolated cases, that there was nothing systematic and that the guilty were in the process of being punished."
The US report said that those involved were low-ranking members of the military and that their acts were not approved by their superiors, the member added.
US has faced criticism from UN human rights experts and international groups for mistreatment of detainees - some of whom died in custody - in Afghanistan and Iraq, particularly during last year's prisoner abuse scandal surrounding the Abu Ghraib facility there.
Some detainees were killed while
in the US-run Abu Ghraib prison
Scores of US military personnel have been investigated, and several tried and convicted, for abuse of people detained during the US-led campaign against so-called terrorist groups.
At the Guantanamo Bay naval base, a US toehold in Cuba where about 520 suspects from 40 countries are held, allegations of torture have combined with other claims of human rights breaches.
The US has faced widespread criticism for keeping the Guantanamo detainees in a "legal black hole," notably for its refusal to grant them prisoner of war status and allegedly sluggish moves to charge or try them.
Washington's report to the committee reaffirms the US position that the Guantanamo detainees are classed as "enemy combatants," and therefore do not benefit from the POW status set out in the Geneva Conventions, the committee member said.
Four UN human rights experts on Thursday slammed the United States for stalling on a request to allow visits to terrorism suspects held at the Guantanamo Bay naval base, and said they planned to carry out an indirect probe of conditions there.
Annan chimes in
Later on Friday, the UN Secretary-General Kofi Annan said that the United Nations' rights experts should have access to the US naval base.
"The Secretary-General is...resolved to allow the experts full access to wherever they need to go"
"The secretary-general is concerned that human rights be applied universally and uniformly and he hopes that this matter can be resolved to allow the experts full access to wherever they need to go," a spokeswoman for Annan, Marie Okabe, said.
Some 520 "war on terror" suspects of some 40 nationalities, captured during the US-led campaign launched after the 11 September 2001 attacks, are being held there, according to the International Committee of the Red Cross, which usually keeps its findings confidential.