Rights group Amnesty International said last month it had documented 101 cases of arbitrary arrest and torture by Cameroonian troops charged with fighting the Nigeria-based jihadist group between 2013 and 2017.

Eighty of the cases occurred at the elite Rapid Intervention Battalion (BIR) headquarters at Salak, in Cameroon's Far North region, which has been hit hard by spillover from the eight-year insurgency in neighboring Nigeria.

French and U.S. troops have been operating out of Salak for several years, with Amnesty saying it had video evidence proving "regular presence of U.S. personnel in numerous locations across the base".

Mark Cheadle, a spokesman for the U.S. military's Germany-based Africa Command, said a "commander's inquiry" had been set up, although he was unable to provide details of its progress.

Boko Haram attacks have killed more than 20,000 people and displaced 2.7 million in northeast Nigeria and adjacent areas of Chad, Niger and Cameroon.