(Bloomberg) -- The presidents of Ukraine and Poland gathered for a church service on Sunday to commemorate an event from 80 years ago that’s among those central to the Polish consciousness — the Volhynia massacre.  

In the final years of World War Two, Ukrainian nationalist units targeted Polish national minorities, killing as many as 100,000 — including many women and children — in a region that was then within the borders of Nazi-occupied Poland but is now in western Ukraine. 

Poland’s parliament passed a resolution in 2016 to declare the actions a genocide. Warsaw has clashed with Kyiv at times over a ban on proposed Polish exhumation works in the region.

Read more: World War II Goes On for Poland and Ukraine: Leonid Bershidsky

Poland has been one of Ukraine’s closest allies since Russia’s invasion in February 2022, providing significant military aid and welcoming millions of refugees. Yet the issue of Volhynia remains contentious, and in an election year, some Polish politicians have called on Kyiv to make an official apology.  

Volodymyr Zelenskiy’s appearance on Sunday may have been a step in that direction. Along with Poland’s Andrzej Duda he attended a memorial service in Lutsk, about 100 kilometers (62 miles) east of the current Polish-Ukrainian border. 

“Memory unites us!” Zelenskiy said on Twitter in a post written both in Ukrainian and Polish. Zelenskiy wrote about honoring “the innocent victims of Volhynia,” without elaborating. 

By contrast, Duda tweeted a video that referenced paying “tribute to the murdered Poles” on what he called Bloody Sunday. 

Ukraine describes the Volhynia tragedy as part of mutual ethnic purges by Poles and Ukrainians.

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