Mateusz Morawiecki added that everything must be done to get to the bottom of that wartime crime in order to be able to “effectively prevent the destructive power of hatred” in the future.
Poland on Thursday marked its National Day of Remembrance of Victims of Genocide by Ukrainian nationalists against Poles during World War II.
In a letter read out during the day’s ceremonies, Morawiecki said: "We pay tribute to Polish citizens murdered ... and we once again make a commitment that we will not rest until the whole truth about this brutal crime is uncovered and until our countrymen—who are today buried in thousands of nameless graves—are commemorated in the right way."
Thursday’s commemorations included a Catholic church service and a roll call of honour.
President Andrzej Duda in the morning placed a wreath at a Warsaw monument that honours victims of wartime killings known as the Volhynia Massacres.
These were carried out between February 1943 and the spring of 1945 by the Ukrainian Insurgent Army (UPA), according to Poland’s National Institute of Remembrance (IPN).
Around 100,000 ethnic Poles in total were slaughtered in the 1940s by Ukrainian forces, according to some estimates.
On July 11, 1943, the day of the worst bloodshed, Ukrainian nationalists attacked 100 villages largely inhabited by Poles in what was then Nazi-occupied eastern Poland and is now western Ukraine.
The massacres were part of an operation carried out by the Ukrainian Insurgent Army (UPA), whose plan was to have a sovereign and nationally homogenous Ukraine after the war.
Morawiecki said in his message on Thursday: “We must also do everything to get to the bottom of the causes of the Volhynia evil in order to effectively prevent the destructive power of hatred in the future.”
The Volhynia region, which was within Poland's borders prior to World War II, was first occupied by the Soviets in 1939, and then by the Nazi Germans in 1941.