Polish Prime Minister Donald Tusk has declared plans for a national memorial honoring victims of a "genocide by Ukrainian nationalists" in World War Two.

Tusk made the statement on the anniversary of the events Poland refers to as the "massacre" in Volhynia, a region in what was German-occupied Poland and is now Ukraine, known as Volyn. Poland estimates around 100,000 ethnic Poles were slain by the Ukrainian Insurgent Army (UPA) between 1943 and 1945.

In Ukraine, many view the UPA as heroes who battled for freedom from the Soviet Union and opposed both Nazi Germany and Polish forces.

The two nations have long disagreed about these incidents, which also resulted in the deaths of up to 10,000 Ukrainian civilians according to estimates.

Addressing the matter on Saturday, Tusk remarked: "Truth is our duty toward the victims, but also a way to overcome a painful past for the sake of a better future.

"Memory cannot be the servant of hatred. The answer to nationalism cannot be more nationalism," the Polish prime minister stated, calling on Ukraine to "embrace this truth" should it seek future European Union membership.

Earlier last month, Ukrainian President Volodymyr Zelensky had Poland's top state honor revoked following his choice to name a Ukrainian military unit after the UPA, sparking a diplomatic dispute between the nations.

Polish President Karol Nawrocki indicated he was compelled to take action, yet emphasized that the disagreement would not affect Poland's backing of Ukraine in its ongoing struggle against Russia's full-scale invasion that began in 2022.

Three former Ukrainian presidents subsequently returned their White Eagle awards to Poland in support of Zelensky.

In a video message delivered late Saturday, Zelensky noted that "representatives of the Ukrainian state took part in joint prayers with representatives of the Polish state" in both countries to remember the victims of the Volyn killings.

"Ukraine is doing its part to honestly establish the facts about those killed in those years.

"We must not forget that now... Ukraine and Poland have one common threat, and this is a mortal threat to our independence, to our states, to every city, to every village, and this threat is called Russia," the Ukrainian president remarked.