The United Nations Committee on Civil and Political Rights has asked the United States to explain the legal status and treatment of prisoners detained in Afghanistan, Guantanamo Bay and Iraq.
The group, which monitors compliance with a 1976 treaty guaranteeing basic freedoms, said Washington was already six years late in filing a regular report on its adherence.
"If a full report can't be done by the end of the year at least they should address ... problems of legal status and treatment of persons detained in Afghanistan, Guantanamo Bay, Iraq and other places of detention outside the United States," Sir Nigel Rodley, a British expert on the committee, told a news briefing.
US forces are holding hundreds of terror suspects in Afghanistan, the US naval base in Guantanamo, Cuba and Iraq , most without charge or legal representation, activists say.
Photographs of US soldiers abusing Iraqi detainees at Abu Ghraib prison near Baghdad sparked world outrage in May.
The committee, which has no powers of sanction, received the last US report in 1994. Reports are generally due every four years on the right to life, self-determination and due process, and freedom of movement, expression and religion in a country.
Rodley, who told journalists the committee had written to the United States, said it was also seeking information on the U.S. Patriot Act, a cornerstone of the U.S. war on terror.
Critics of the act, including the American Civil Liberties Union, say that it gives the FBI unchecked powers of surveillance at home.
A spokeswoman at the US diplomatic mission in Geneva said that Washington intended to file its report in the next few months. "This follow-up response will address questions on all relevant activities of the U.S. armed forces," Brooks Robinson told Reuters.