Polish Prime Minister Donald Tusk announced the creation of a national monument to the victims of a “genocide committed by Ukrainian nationalists” during World War II.

Tusk spoke on the anniversary of what Poland calls the “massacre” in Volyn, the part of Poland then occupied by the Germans, now in Ukraine. Warsaw says that some 100,000 ethnic Poles were killed by the Ukrainian Insurgent Army (UPA) between 1943 and 1945.

Warsaw and kyiv have been at loggerheads for decades over tragic events, in which thousands of ethnic Ukrainians were also murdered by Poles.

Many in Ukraine see the UPA as heroes who fought for Ukrainian independence against the Soviet army, as well as Nazi Germany and Polish authorities.

Last month, Ukrainian President Volodymyr Zelensky was stripped of Poland’s highest state honor over his decision to name a Ukrainian military unit after the UPA, amid a diplomatic dispute between the two countries.

Speaking on Saturday, Tusk said: “The truth is our duty to the victims, but also a way to overcome a painful past for a better future.

“Memory cannot be at the service of hate. The answer to nationalism cannot be more nationalism,” the Polish prime minister said, urging Ukraine to “embrace this truth” if the country wanted to one day join the European Union.

Polish President Karol Nawrocki said he had to act, but stressed that the dispute would not affect Poland’s support for Ukraine as it continues to fight a large-scale Russian invasion launched in 2022.

Ukrainian Foreign Minister Andrii Sybiha responded to Warsaw’s move by calling it a “strategic mistake.”

Three former Ukrainian presidents subsequently returned their White Eagle awards to Poland in solidarity with Zelensky.

In his video speech late Saturday, Zelensky said that “representatives of the Ukrainian state participated in joint prayers with representatives of the Polish state” in both countries to commemorate the victims of the Volyn murders.

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

“We must not forget that now… Ukraine and Poland have a common threat, and this is a deadly threat to our independence, to our states, to every city, to every people, and this threat is called Russia,” the Ukrainian president said.