DAKAR (Reuters) - Cameroonian forces have been torturing suspects in a campaign against Islamist group Boko Haram, with much of the torture happening at a base used extensively by American troops and visited by French ones, Amnesty International said on Thursday.

Amnesty’s report documented 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 Boko Haram has been fighting for the past eight years to recreate an 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 U.S. Africa Command said it had not received any reports of human rights abuses by Cameroonian forces at the base mentioned.

France’s Defence and Foreign ministries declined to comment. A French diplomat told Reuters that French troops hadn’t participated in any abuses but that French forces had used the bases.

A Cameroon defence ministry spokesman accused Amnesty of “bad faith” and of trying “to transform killers into victims”.

BEATINGS, SHOCKS

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,” said Colonel Didier Badjeck, Cameroon’s army spokesman, 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 said it had observed French troops at the base in May 2015. It 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.”

Amnesty urged both nations to investigate whether or not their assistance “has contributed to the commission of these crimes and violations.”

“French military personnel have certainly visited ... Salak, but have not been present to the same degree as the Americans, and may not have the same degree of potential knowledge,” Steve Cockburn, Amnesty’s deputy regional director for West and Central Africa, told Reuters.

“However, they remain an important military partner for Cameroon and they should investigate whether their personnel did have any knowledge of torture being carried out,” he said.

Robyn Mack, a spokeswoman at the U.S. military’s Africa Command, said it had not received any reports of human rights abuses by Cameroonian forces at either of these locations, and that all military assistance included human rights law training.