The revelation comes as a CIA contractor is charged with assaulting an Afghan detainee who later died of his injuries.
The secrecy surrounding the centres makes "inappropriate detention and abuse not only likely, but inevitable", said the New York-based Human Rights First in a report on Thursday.
The centres are in Iraq, Cuba, Afghanistan, Pakistan, Jordan and on two US ships, said the human-rights group. They fail to meet obligations under US and international law on the treatment of prisoners, said the report entitled Ending Secret Detention.
It was released on the same day that US Defence Secretary Donald Rumsfeld acknowledged that the occupation authority had secretly held a prisoner in Iraq and failed to register the detainee with the International Committee of the Red Cross (ICRC).
Rumsfeld refused to cite the reason for the secrecy, saying it was classified. However, he denied it was done to prevent international monitors from gaining access to the suspect.
"The United States government is holding prisoners in a secret system of off-shore prisons beyond the reach of adequate supervision, accountability or law"
Director of Human Rights First's US Law and Security Programme
The report's release also followed the publication of photos of the sexual abuse and humiliation of Iraqi detainees at the hands of occupation soldiers at Abu Ghraib prison outside Baghdad and reports of abuse at the US base at Guantanamo Bay, Cuba.
"The abuses at Guantanamo Bay and Abu Ghraib cannot be addressed in isolation," said Deborah Pearlstein, director of Human Rights First's US Law and Security Programme.
"The United States government is holding prisoners in a secret system of off-shore prisons beyond the reach of adequate supervision, accountability or law."
Among the detention camps that the US government refuses to disclose but have been reported to Human Rights First by "multiple sources" are a centre in Kohat, Pakistan, near the Afghan border; al-Jafr Prison, a US Central Intelligence Agency interrogation facility in Jordan; and a facility on the Indian Ocean island of Diego Garcia.
The ships USS Bataan and USS Peleliu were also suspected detention sites, said the report by the rights group.
Secrecy makes detainee abuse
'not only likely but inevitable'
Most of the detention centres listed in the report were in Iraq, including Abu Ghraib; Camp Cropper near Baghdad International Airport; Camp Bucca near Basra; and nine centres run by military division or brigades.
The report said suspected sites were also in Afghanistan, including CIA interrogation facilities in Kabul and at Bagram Air Force Base.
The other sites are known, including a collection centre at Bagram; a facility in Kandahar; the Guantanamo Bay base; and a US military brig in Charleston, South Carolina.
Human Rights First called on Washington to end secret detentions; notify the families of the detainees; investigate abuses; implement preventative measures; release the location of the detention facilities; and give the Red Cross immediate access to all detainees.
The Red Cross has been given access to some detainees, most notably ousted Iraqi President Saddam Hussein.
In related developments, a former Army Ranger hired by the CIA to conduct interrogations was charged with assaulting an Afghan detainee who died after two days of beatings, the first time civilian charges have been brought in the investigation of prisoner abuse in Iraq and Afghanistan.
A four-count grand jury indictment was handed up Thursday in the US state of North Carolina, against David Passaro, 38, for the 21 June 2003, killing of Abd al-Wali.
Attorney General John Ashcroft said Passaro was accused of "brutally assaulting" Wali at a US base in Asadabad, Afghanistan.
Asked why Passaro was not charged with torture or other more serious offences, Ashcroft said the indictment was based on the best evidence available. He said more serious charges could be brought if new evidence is found.