Two Yemeni men claim they were held in secret, underground US jails for more than 18 months without being charged, Amnesty International has said.
Both men say they were held in underground jails run by US guards
The human rights group has called on the US to reveal details of the alleged secret detention of suspects abroad.
Amnesty fears the case is part of a "much broader picture" in which the US holds prisoners at secret locations.
The US has not responded to the claims, but the head of the CIA recently said the agency does not use torture.
Porter Goss said in testimony to the US Senate torture was neither professional nor productive.
Beaten on feet
In the new report, Amnesty has urged the US to reveal where its alleged secret detention facilities are, stop using them and name the detainees held there.
The two Yemeni men, Muhammad Faraj Ahmed Bashmilah and Salah Nasser Salim Ali, were arrested separately but reported almost identical experiences to Amnesty.
Mr Muhammad says he was arrested in 2003 in Jordan, while Mr Salah says he was detained in Indonesia the same year and later flown to Jordan.
Both say they were tortured for four days by Jordanian intelligence services.
Alleged methods include being beaten on the feet while bound and suspended upside-down. One of the men claims he was threatened with sexual abuse and electric shocks.
Each says he was then flown to an unnamed underground jail, where he was held in solitary confinement for six to eight months with no access to lawyers.
Both claim they were interrogated every day by US guards about their activities in Indonesia and Afghanistan.
They say a period in a second underground prison followed, where loud Western music was piped into the cell 24 hours a day and questioning by US officials continued.
The men were transferred in May this year to Yemen, where they are still being held without charge.
Amnesty says the Yemeni authorities say they are only holding the men because the US has "made it a condition of their release from secret detention".
Bashmilah says he was arrested in Jordan while visiting his sick mother
Amnesty's Sharon Critoph, who interviewed the men in Yemen, said: "To be 'disappeared' from the face of the earth without knowing why or for how long is a crime under international law and an experience no-one should have to go through.
"We fear that what we have heard from these two men is just one small part of the much broader picture of US secret detentions around the world."
Michael Ratner, of the US campaign group Center for Constitutional Rights, said the report was the first to touch on the "netherworld of secret detention facilities that the CIA is running".
Amnesty has previously reported on what it calls the long-term detention without trial or charge of prisoners in Yemen at the request of US authorities.
The US has also faced questions over its use of "rendition", a process by which terror suspects are sent for interrogation by security officials in other countries, some of which are accused of using torture.