Moscow, Russia -- A Russian court on Monday sentenced a 72-year-old United States citizen accused of fighting as a mercenary on the side of Ukraine to nearly seven years in prison.

Russia has recently detained and tried a number of US citizens and completed a large prisoner exchange, including Wall Street Journal reporter Evan Gershkovich, which enabled the return of Russians jailed in the United States.

On Monday, a court in Russia's western city of Voronezh also sentenced a US man already serving a jail term to a further seven years on a charge of violence in prison.

At Moscow City Court, judge Alexandra Kovalevskaya sentenced the 72-year-old defendant, named as Stephen Hubbard by the media, to six years and 10 months, an AFP journalist heard.

The bearded defendant dressed in a knitted jacket and dark trousers shuffled slowly into court and stood with difficulty as the sentence was read out.

He was convicted of "participating as a mercenary in the armed conflict" after a brief trial largely held behind closed doors.

The sentence took into account the fact that Hubbard has already been in custody since 2 April 2022.

Every day he spent in pre-trial detention subtracts 1.5 days from his sentence, meaning he has already served over 3.5 years, RIA Novosti news agency reported.

The lawyer representing Hubbard told journalists he was likely to appeal against his sentence.

Hubbard had risked up to 15 years in prison but prosecutors asked for the minimum of seven years taking into account his age and guilty plea, the judge told RAPSI news agency.

His case only became public on 27 September, when his trial began in Moscow. Russia has not said where he had been detained.

Hubbard appeared at a hearing last week, when the court ordered that the trial be held in secret without the media, at the request of prosecutors.

Prosecutors said that Hubbard was paid at least $1,000 a month to join a Ukrainian territorial defense unit.

They say he underwent training, was given a combat uniform and "took part in the armed conflict" in Ukraine.

Russia's state-run TASS news agency said Hubbard had been living in the Ukrainian city of Izyum in the northeastern Kharkiv region since 2014.

Russian forces took control of the city of 45,000 shortly after ordering troops into Ukraine, before being ousted in September 2022 in a lightning counter-offensive by Kyiv.

Russia has not given any details on the circumstances of Hubbard's arrest.

A video posted on pro-Russian YouTube channels in May 2022 -- during the Russian occupation of Izyum -- showed a bearded man who gave his name as Stephen James Hubbard, said he was born in Big Rapids, Michigan, and came to live in Ukraine in 2014.

The other US citizen convicted in Russia on Monday, named as Robert Gilman, was handed a term of seven years and one month in a strict-regime penal colony.

He was found guilty of attacking prison staff and a criminal investigator, Russian news agencies reported.

He had been convicted in 2022 of attacking a policeman while drunk in the city of Voronezh and sentenced to four years and six months in prison, later reduced to three and a half years on appeal.

While in jail, he punched members of prison staff "in the head" on two separate occasions and attacked a criminal investigator, according to prosecutors.

Russia has arrested numerous Westerners in recent years on charges ranging from espionage to petty theft, with some cases related to Moscow's Ukraine offensive.

They include Ksenia Karelina, a dual US-Russian citizen who was arrested while visiting family in Russia and sentenced to 12 years in jail for donating around $50 to a Ukrainian organization.

Two Colombian citizens are also being held in Russia on charges of being "mercenaries" for Ukraine.

Russia and the West held the largest East-West prisoner swap since the Cold War on 1 August. Journalists and Russian dissidents jailed on one side were swapped for Russians imprisoned for murder, espionage and other crimes on the other.