Former detainees and Yemeni officials have claimed that the United Arab Emirates transferred terror suspects from Yemen to a secret prison in Eritrea.
The prison in the city of Assab on the Red Sea coast is part of a regional network in which inmates are tortured and abused, Associated Press and Human Rights Watch investigations have revealed.
HRW investigators spent six months documenting alleged abuses against prisoners in Yemen.
According to HRW researcher Kristine Beckerle, the UAE and its Yemeni associates use the prisons for "high-value terrorism suspects [who] are accused of a variety of things related to the UAE efforts against al-Qaida in Yemen".
Her team of researchers found that some detainees were transferred to Assab, where the UAE has a military base and holds an unknown number of prisoners.
"There's been absolutely no transparency or communication either with the families or with independent monitors," Beckerle told Voice of America. "We've got a whole bunch of prisons that nobody has access to, other than the forces that are running them and detainees that are in them."
HRW alledge that the UAE - which has been fighting al-Qaeda in Yemen since 2015 - have been involved in the torture of informal detainees in both Yemen and in Eritrea.
The rights watchdog recorded the cases of 49 people, including four children, who were detained or forcibly disappeared. HRW says 38 of the 49 were arrested by UAE-backed forces.
Relatives of prisoners told HRW that beatings involved punches, metal instruments, electic shocks, canings and sexual abuse.
"The UAE was backing Yemeni forces who were implicated in a lot of these abuses," Beckerle said. "And the UAE itself was involved in these detention campaigns, including running some informal detention facilities, and ordering continued detention of people despite release orders, and transferring people potentially out of Yemen to different places for detention."
Eritrea's Minister of Information Yemane Gebremeskel has vehemently denied HRW's findings and has called the reports "patently false".
US senators, however, have taken notice of the allegations and called for an investigation into Washington's potential involvement in torture in Yemen.
Those leading the calls for the investigation include senators John McCain of Arizona and Jack Reed of Rhode Island.
US operations in Yemen involve the provision of logistical and intelligence support to the Saudi-led military coalition which the UAE is part of.
According to AP's investigation, US personnel were involved in interrogations in Yemen, however no evidence of torture was reported.
In response to the allegations, Yemen's internationally-recognised government also recently announced the creation of a committee to investigate allegations of human rights violations.
