Amnesty International is calling for an independent investigation into the torture of prisoners in Cameroon.
The human rights group said Cameroonian forces have been torturing suspects in their campaign against Islamist group Boko Haram, with much of the torture happening at a base that has also been used by American and French troops.
An Amnesty report documents 101 cases of arbitrary arrest and torture by Cameroonian troops charged with fighting the insurgents between 2013 and 2017.
Some of the victims were tortured to death, it said.
The Nigerian militant group has been fighting for the past eight years to create a medieval Islamic caliphate around Lake Chad, where Nigeria, Cameroon, Niger and Chad meet. Boko Haram attacks have killed more than 20,000 people and displaced 2.7 million in the region, according to aid agency figures.
Atrocities such as the kidnapping of more than 200 school girls from the Nigerian village of Chibok in 2014 persuaded Western countries, especially the United States and France, to provide counter-insurgency assistance to some of the countries affected, including intelligence and training.
The US Africa Command said it had not received any reports of human rights abuses by Cameroonian forces at the base mentioned.
French Defence Ministry officials did not immediately comment.
A Cameroon defence ministry spokesman accused Amnesty of “bad faith” and of trying “to transform killers into victims”.
The torture techniques, which Amnesty described as “chilling”, include a “stress position described as ‘the goat’: the detainee’s arms and legs are tied together behind his back and he is left on the ground and beaten.”
“In a common suspension technique known as ‘the swing’, the victim’s arms and legs are again tied behind his back, before he is lifted and suspended on a bar fitted between two poles … and further beaten,” Amnesty said.
Other torture documented included being tied standing up in stress positions for 24 hours, being subject to simulated drowning, being deprived of food, forced to drink urine, given electric shocks and burned.
Victims included women, the disabled and the mentally ill, the report said.
“Our army is professional and disciplined,” Cameroon’s army spokesman Colonel Didier Badjeck said, reacting to questions from Reuters. “It has better things to do than to spend time justifying itself against people who have preconceived ideas.”
Eighty of the 101 cases of torture took place at the elite Rapid Intervention Battalion (BIR) headquarters at Salak, in Cameroon’s Far North region, the heart of the insurgency, Amnesty reported.
Amnesty said its delegates had observed French troops at the base in May 2015. It also said it had still and video images “clearly showing the regular presence of U.S personnel in numerous locations across the base, including making use of a makeshift gym and a trailer converted into an office.
It urged the United States and France to investigate whether their military personnel knew that torture was taking place on the site, and whether or not their assistance “has contributed to the commission of these crimes and violations.”
“To date, US Africa Command has not received any reports of human rights abuses by Cameroonian forces at either of these locations,” Robyn Mack, a spokeswoman at the U.S. military’s Africa Command, said in a statement.
“Any foreign military unit that receives security assistance receives training on the law of armed conflict and human rights law.”