Polish Prime Minister Mateusz Morawiecki said on Wednesday reconciliation with neighbor Ukraine could take place only when Kiev accepts responsibility for the "genocide" of Poles killed by Ukrainian nationalists during World War Two.
Morawiecki was marking an anniversary of the 1943-45 killings in Wolyn, an area that was part of Poland before the war and is part of Ukraine now. Soviet and Nazi troops both rolled through the fertile lands in the tumultuous years.
Poland says 100,000 Poles were killed in the area during the Nazi occupation by Ukrainian nationalists hoping to set up an ethnically pure state. It says smaller numbers of Ukrainians were killed in reprisals.
Ukraine says large scale killings took place on both sides, and tends to avoid comparing numbers. President Petro Poroshenko expressed regret over the "fratricidal" conflict on Sunday when he visited a Polish village where Ukrainians were killed.
The painful history haunts current-day relations between Poland, which cast off post-war communism in 1989 and is now a member of the EU and NATO, and ex-Soviet Ukraine, which now counts on Warsaw to promote its Western aspirations.
"These wounds, these wounds of memory, these wounds of forgetting can only be healed on the foundation of truth," Morawiecki told a ceremony in Warsaw. "Only based on the foundation of truth we could build a future reconciliation."
Both Ukraine and Poland have seen surges of nationalist sentiment in recent years, harking back to the war era.
In Ukraine, pride in the war-time independence movement has risen since 2014, when Russia annexed Crimea and backed an armed revolt in east Ukraine, a conflict that has killed 10,000 people and still simmers.
In Poland, the rightwing Law and Justice (PiS) party won elections in late 2015 and has campaigned on pledges to address historical grievances.
It angered Israel and the United States by passing a law that would have imposed jail terms for suggesting that Poland was complicit in the Nazi Holocaust of Jews. That part of the law has been withdrawn, but another part referring to the "genocide" of Poles by Ukrainians remains in place.
According to Kiev, Poland's President Andrzej Duda declined to hold joint commemorations of the 1943-45 events together with his Ukrainian counterpart in what Ukraine hoped would have been a sign of reconciliation and unity toward Russia.
In the end, Duda went to western Ukraine, while Poroshenko visited Poland last week, where he lay flowers at a memorial for Ukrainian villagers killed by Poles. He emphasized that Ukrainians and Poles often fought side by side against common enemies, listing conflicts when they battled Russia together.
"But we have to admit that there are sad pages in our history as well. Along with the common victories, along with our friendship and fraternity, the fratricidal armed Polish-Ukrainian conflict of 1943-1944 remains the most painful trauma," he said. He called on Poland not to let common enemies use history to divide the two countries.
Despite the high-level tension, more than a million Ukrainian workers have moved to Poland in recent years, and the people of both countries generally remain friendly.
"They owe us an apology but this should not turn drastic. We have a good alliance with Ukraine," said a retired Polish cook, Jadwiga Rzepka.