A Russian court on Monday sentenced 72-year-old American citizen Stephen James Hubbard to six years and 10 months in prison after convicting him in a closed-door trial of serving as a mercenary for Ukraine, a Reuters reporter in the courtroom said.

Investigators said Hubbard, a native of Michigan, was paid $1,000 per month to serve in a Ukrainian territorial defence unit in the eastern city of Izyum, where he had been living since 2014.

The RIA state news agency quoted the prosecutor as saying last month that Hubbard was detained by Russian soldiers on April 2, 2022, just weeks after Moscow sent thousands of troops into Ukraine.

Russian state media said Hubbard had pleaded guilty to the charge. But in interviews last month, Hubbard's sister and another relative cast doubt on his reported confession, telling Reuters he held pro-Russian views and was unlikely to have taken up arms at his age.

"He is so non-military," his sister Patricia Hubbard Fox told Reuters by phone last month. "He never had a gun, owned a gun, done any of that...He's more of a pacifist."

Reuters was unable to confirm how Hubbard was detained. His lawyer could not immediately be reached. The Ukrainian Foreign Affairs Ministry has not replied to multiple messages seeking comment.

A spokesperson for the U.S. embassy in Moscow has said it was aware of the detention of an American citizen, but declined further comment.

Hubbard is one of at least 10 Americans behind bars in Russia, nearly two months after a prisoner swap on Aug. 1 between Moscow and the West freed three Americans and dozens of others.