US forces have been involved in interrogation of hundreds of inmates in clandestine prisons run by the United Arab Emirates (UAE) in war-torn Yemen, with reports of brutal torture and abuses at the facilities.

The Associated Press documented at least 18 secret jails across southern Yemen run by the UAE or by Yemeni militia loyal to the former Yemeni government, where prisoners face extreme abuse and torture on a routine basis.

On Wednesday, senior US defense officials confirmed that the American forces have been involved in interrogations of detainees in Yemen but denied any participation in or knowledge of human rights violations.

Several torturing methods are being used at the jails, including the “grill” in which the victim is tied to a spit like a roast and spins in a circle of fire, according to the report.

Former inmates released from one main detention facility at Riyan airport in the southern city of Mukalla, said they were crammed into shipping containers covered with feces and blindfolded for weeks. They said they were beaten, trussed up on the “grill,” and sexually abused.

“The entire place is gripped by fear. Almost everyone is sick, the rest are near death. Anyone who complains heads directly to the torture chamber,” said a former detainee held for six months at Riyan airport.

So far, over 400 men have disappeared after being swept up in Mukalla.

The UAE secret jail network in Yemen was established during former US president Barack Obama’s administration and still continues its operations, according to the report.

In another report on Thursday, Human Rights Watch said the UAE runs two “informal detention facilities” in southern Yemen and has “moved high-profile detainees outside the country,” including to a base in Eritrea.

The New York-based rights group said it had documented 49 cases, including those of four children, who had been “arbitrarily detained or forcibly disappeared,” most of them by UAE-backed forces.

In a statement to the AP, the UAE’s government denied the allegations, saying, “There are no secret detention centers and no torture of prisoners is done during interrogations.”

However, lawyers and families say nearly 2,000 men have disappeared into the clandestine prisons across Yemen. The issue has triggered days of protests by families seeking information about missing sons, brothers and fathers.

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Several US defense officials, speaking on condition of anonymity, told the AP that American forces participate in interrogations of detainees in Yemen, provide questions for interrogators and receive transcripts of interrogations from Emirati forces.

They said senior US military leaders have looked into the allegations of torture at the clandestine prisons in Yemen, but were satisfied that there was no case of abuse when the US forces were present.

However, a member of the Hadramawt Elite, a Yemeni security force set up by the UAE, told AP that American forces were sometimes only yards away from the scene of torture.

According to international law experts, obtaining intelligence that may have been extracted by torture inflicted by another party is considered as violation of the International Convention Against Torture and constitutes an instance of war crime.

The UAE has served as an ally of Saudi Arabia in the latter’s US-backed campaign in Yemen to restore the impoverished country’s former Riyadh-allied government. The Elite Forces have been fighting in Yemen since the same year to assist the Saudi-led campaign.

According to various reports, Abu Dhabi holds notable sway in southern Yemen and looks to be trying to expand its leverage there by lending its support to southern separatists.

The separatists are led by two pro-Emirati officials of Yemen’s former president Hadi, who have been sacked by him over suspicions of serving the Emirates.

In March 2015, the Saudi regime and its allies began the campaign against Yemen to reinstall its former government. The war has killed over 12,000 civilians. The invasion has been compounded by a Saudi blockade of the country.