Polish Prime Minister Donald Tusk has announced the creation of a national memorial to the victims of a "genocide committed by Ukrainian nationalists" during World War Two.

Tusk was speaking on the anniversary of what Poland calls the "massacre" in Volhynia - a Polish territory in German-occupied Poland and now known as Volyn and part of Ukraine. Warsaw says some 100,000 ethnic Poles were killed by the Ukrainian Insurgent Army (UPA) in 1943-45.

Warsaw and Kyiv have for decades been at odds over the tragic events.

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.

Speaking on Saturday, Tusk said: "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 said, urging Ukraine to "embrace this truth" if the country wanted one day to join the European Union.

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

Polish President Karol Nawrocki said he had to act - but stressed that the row would not impact Poland's support for Ukraine as it continues to fight against a full-scale Russian invasion launched in 2022.

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

In his video address late on Saturday, Zelensky said that "representatives of the Ukrainian state took part in joint prayers with representatives of the Polish state" in both countries to commemorate 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 said.