A court in Finland found a Russian paramilitary fighter guilty of war crimes committed during Russia’s first invasion of Ukraine, sentencing him to life in prison on Friday.

Voislav Torden, a Russian citizen who also goes by the name Yan Petrovskiy, was charged with committing war crimes for leading an ambush on Ukrainian soldiers in 2014, when Russian-backed forces first invaded Ukraine’s eastern front, according to the court ruling.

It was the first time a court in Finland presided over a case involving analleged international war crime committed during the conflict in Ukraine, and a rare instance of a conviction for war crimes carried out when Russian forces first crossed into Ukraine more than a decade ago. In 2022, a United Nations-led commission of inquiry concluded that Russian forces had committed widespread atrocities in eastern Ukraine.

Mr. Torden had pleaded not guilty to five counts of committing war crimes, but a panel of three judges unanimously found him guilty of leading the ambush, murder, mutilation and distributing harmful images online. He was acquitted of a charge linked to planning the ambush because of insufficient evidence, the court said in its ruling.

Mr. Torden was a leader of Rusich, a neo-Nazi militia group that fought alongside Russia’s military and is associated with Wagner, the Russian private military company, according to the United States government. He and other members of the militia had been sanctioned by the United States, as well as the European Union and other allied countries.

Rusich mercenaries are known to have fought alongside Russian-backed proxy forces in the Donbas region in 2015 and to have appeared again on the battleground surrounding Ukraine’s northeastern city of Kharkiv in 2022, according to the U.S. Department of the Treasury.