“Yes, I agree with the indictment,” RIA cited him as saying.

Hubbard, 72, was placed in pre-trial detention last week for six months. He faces a sentence of seven to 15 years if convicted.

RIA, citing a prosecutor in court, said Hubbard had signed a contract with a Ukrainian territorial defence unit in the central city of Izyum at the start of the war in February 2022.

The prosecution said he was promised US$1,000 a month and was provided with training, weapons, and ammunition. Hubbard was detained by Russian soldiers on April 2 of that year, RIA quoted the prosecutor as saying.

Hubbard, a native of Michigan, had worked as an English teacher abroad for decades, his sister, Patricia Fox, told Reuters. She denied her brother was a mercenary and said he had no interest in fighting in any war.

She said Hubbard had moved to Ukraine in 2014 and lived there for a time with a woman, surviving off a small pension.

Hubbard is one of at least 10 Americans currently behind bars in Russia.