A US citizen has pleaded guilty in a Moscow court to charges of fighting for Ukraine, marking the first known instance of an American being tried as a mercenary in Russia.

Russian state media reported that Stephen James Hubbard, 72, of Michigan, had admitted he had received money to fight for Ukraine against Russia.

“Yes, I agree with the indictment,” the state news agency Ria Novosti quoted him as saying in court on Monday.

The Kremlin-friendly Izvestia newspaper, citing a prosecutor in court, said Hubbard was paid $1,000 (£750) after he signed a contract with a Ukrainian territorial defence unit in the eastern city of Izium in February 2022.

He was captured by Russian forces in April 2022, according to the prosecutor, during the occupation of Izium by Russian troops. The city was later liberated by Ukrainian forces in the autumn of 2022.

If convicted of participating in mercenary activities, he could face up to 15 years in prison.

Hubbard’s sister Patricia Fox denied her brother was a mercenary and said he was too old for combat. “He is so non-military,” Fox told Reuters. “He never had a gun, owned a gun, done any of that … He’s more of a pacifist.”

In a public Facebook group, Fox earlier said that her brother had been “kidnapped up in the Ukraine” nearly three years ago.

Hubbard has reportedly spent decades working abroad as an English teacher, including in Japan, Cyprus and Ukraine, where he was living at the time of Moscow’s invasion in February 2022.

A spokesperson for the US embassy in Moscow said last week that it was aware of the detention of an American citizen, but declined further comment.

Hubbard is one of at least a dozen Americans behind bars in Russia. Arrests of Americans in Russia have become increasingly common as relations between the two countries sink to cold war-era lows. Washington has previously accused Moscow of using US citizens as bargaining chips for political leverage.

Hubbard is the first known US citizen to be tried in Russia on mercenary charges in relation to the war in Ukraine.

In June 2022, two American men were captured by Russian-backed separatist forces in Donbas while fighting alongside Ukrainian forces during a battle north of Kharkiv. The men were later released along with five British citizens as part of a prisoner swap between Russia and Ukraine that was brokered by Saudi Arabia.