Stephen James Hubbard, a 72-year-old U.S. citizen, pleaded guilty in a Moscow court on Monday to charges of serving as a mercenary against Russia in the Ukraine war, Russia's RIA news agency reported, Paralel.Az reports.

Hubbard's sister and another relative quickly cast doubt on his reported confession, telling Reuters he held pro-Russian views and was unlikely to have taken up arms at his age.

RIA said Hubbard had admitted in a hearing at the Moscow City Court that he had received money to fight for Ukraine against Russia.

"Yes, I agree with the indictment," RIA quoted him as saying.

Hubbard 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 eastern Ukrainian city of Izyum at the start of the war in February 2022.

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