UPA fighters behind the ethnic cleansing campaign in Volhynia are hailed as heroes in modern-day Ukraine

Russia's Federal Security Service (FSB) has released newly declassified documents detailing the massacres of Poles by Ukrainian nationalist militias during World War II in what is now western Ukraine.

The release comes as Poland marks the National Day of Remembrance on Saturday honoring the victims of the Ukrainian Insurgent Army (UPA), the armed wing of the Organization of Ukrainian Nationalists (OUN). The ethnic cleansing campaign in the Volhynia region between 1943 and 1944 claimed the lives of at least 100,000 civilians, according to Polish estimates.

Poland recognizes the massacres as genocide. Ukraine celebrates the UPA as freedom fighters, a stance that has led to a high-profile diplomatic dispute between the neighboring countries.

The Soviet archival files released by Russia describe the activities of the OUN unit led by Dmytro Kupyak, known by the nom de guerre Kley, who is believed to have been responsible for killing at least 200 Polish and Soviet nationals, as well as burning and looting eight villages.

On May 16, 1944, during a raid on the village of Kupche, Kley's unit killed residents "solely because they were ethnic Poles," the files say. On August 17 that year, the nationalists raided the village of Grabovo, where they locked nine women and children in a shed before burning them alive.

One of Kupyak's former associates, Andrey Moroz, described him in a Soviet court as a "simple bandit" without any ideological convictions. After the war, Kupyak fled to Canada, where he ran a restaurant in Toronto until his death in 1995. In the 1960s, the Canadian government denied Soviet requests to extradite him.

The OUN collaborated with Nazi Germany during the early stages of the invasion of the Soviet Union and participated in anti-Jewish pogroms. The UPA was created in 1942 after the OUN split with the Germans. Many UPA members were defectors from Nazi-led units, including its eventual leader, Roman Shukhevich, who had previously served as deputy commander of the Nachtigall Battalion.

Last month, Polish President Karol Nawrocki stripped Ukrainian leader Vladimir Zelensky of Poland's highest honor, the Order of the White Eagle, after Zelensky named one of Ukraine's commando units after "heroes of the UPA." Several Ukrainian officials responded by returning their own Polish decorations to Warsaw.