Stephen Hubbard, 72, was accused of signing a $1,000-per-month contract with a Ukrainian territorial defence unit in the city of Izyum in February 2022.

He was captured by Russian forces in April that year, shortly after the war began.

It is unclear when or how he arrived in Moscow.

After a trial was held behind closed doors in Moscow's highest city court, journalists were allowed to witness the verdict.

Hubbard, from Michigan, looked fragile and appeared to be limping as he entered the courtroom.

"He doesn't look like a mercenary," a court employee was overheard saying, as the handcuffed pensioner walked in, carrying a white plastic bag.

Dressed in a black woolly hat, a white jumper, dark trousers and black sandals, Hubbard was guided by bailiffs into the defendant's glass cage, known as the "aquarium".

He removed his hat when the judge began to deliver her verdict and appeared to struggle to stand up.

Hubbard moved to Ukraine in 2014, according to Russia's RIA news agency.

Prosecutors said he was provided with training weapons and ammunition when he allegedly signed up to fight.

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His detention comes two months after a prisoner swap between Moscow and the West saw 24 people exchanged, including three Americans.

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

A US embassy spokesperson said: "We are aware of reports of the arrest and prosecution of an American citizen. Due to privacy restrictions we are unable to comment any further."