The man, identified as Stefan Hubbard from the state of Michigan, appeared in court wearing a black hat and beige jacket, walking slowly and dragging his feet, an AFP journalist reported from the court.

He faces up to 10 years in prison over “participating as a mercenary in the armed conflict on the side of Ukraine,” Russian news agencies reported.

At the start of a hearing in Moscow, prosecutors requested the trial be moved behind closed doors — standard practice for high-profile or politically sensitive trials in Russia.

The defence did not contest the request and the judge asked journalists and US embassy officials to leave the courtroom.

Prosecutors allege that Hubbard was paid at least US$1,000 a month to join a Ukrainian territorial defence unit. They say he underwent training, was given a combat uniform and “took part in the armed conflict” in Ukraine.

They say he was captured on April 2, 2022, weeks after Russia launched its military offensive.

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 shortly after ordering troops into Ukraine, before being ousted in September 2022 in a lightning counter-offensive by Kyiv.

Russia has arrested several US citizens in recent years, including over alleged offences related to its Ukraine offensive.

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

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