MOSCOW — 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 defense unit in the eastern city of Izyum, where he had been living since 2014.

They said Hubbard was provided with training,…