There are reliable reports of prisoners being tortured at the U.S. base in Guantanamo Bay, Cuba, United Nations human rights investigators said Thursday.

They say many of the allegations come from declassified United States government documents.

The investigators also accused the U.S. of stalling on their request to visit foreign detainees to check on their condition. They made their request in April.

While Washington has allowed the International Committee of the Red Cross to visit Guantanamo, it says it is still considering the UN's request.

Red Cross investigators report their findings only to the detaining power.

The UN human rights experts said they want to visit the base because of "information, from reliable sources, of serious allegations of torture, cruel, inhuman and degrading treatment of detainees, arbitrary detention, violations of their right to health and their due process rights."

Washington says it is holding 520 men in Guantanamo, most of whom were captured in Afghanistan during the fall of the Taliban regime in late 2001. Only four have been charged.

Human rights activists say it's unacceptable that many of those arrested since the Sept. 11, 2001 attacks in the U.S. have been held for more than three years without charges and often incommunicado.