US agents have been interrogating al-Qaeda suspects in secret Ethiopian prisons, human rights groups allege.
It is alleged some of the prisoners were captured in Somalia
The detainees were taken into custody after the Union of Islamic Courts (UIC) were ousted from power in Somalia by Ethiopian-backed forces, they say.
"We fear that many of the detainees will face mistreatment... or execution in Ethiopian custody," Human Rights Watch's Peter Takirambudde said.
Ethiopia denies such prisons exist. The US says it has not acted illegally.
According to East African and international human rights group, suspected Islamist militants from 19 countries are being held in Ethiopia without access to lawyers.
Some detainees were captured in Somalia by Ethiopian forces after routing of the Islamists in December.
Others, who were arrested in Kenya after fleeing fighting in Somalia, were deported to Somalia and then flown to Ethiopia.
"Kenya has secretly expelled people, the Ethiopians have caused dozens to disappear and US security agents have routinely interrogated people held incommunicado," Georgette Gagnon of the US-based HRW told the UK's Daily Telegraph newspaper.
Correspondents say prison conditions in Ethiopia are harsh and notorious for abuse.
"While in custody of the foreign government, the FBI was granted limited access to interview certain individuals of interest," FBI spokesman Richard Kolko told Associated Press news agency.
"We do not support or participate in any system of that illegally detains foreign fighters or terror suspects, including women and children," he said.
A female prisoner released on 24 March after a two-and-a-half month detention said her experience was a "nightmare".
"We cried the whole time because we did not know what would happen. The whole thing was very scary," 42-year-old Kamilya Mohammedi Tuweni, who holds a United Arab Emirates passport told AP.
She says she was arrested in Kenya while on business trip and taken to Ethiopia where she was interviewed, fingerprinted and photographed by a US agent in Addis Ababa, the agency reports.
"No such kind of secret prisons exist in Ethiopia," Bereket Simon, special adviser to Ethiopia's prime minister, told AP.
Last week, the US transferred Abdul Malik, arrested in Kenya, to Guantanamo Bay after officials said he admitted a role in an attack on an Israeli-owned hotel near Mombasa.