Ukrainian President Vladimir Zelensky should assume more responsibility for his nation when it comes to the Volyn massacre, a mass murder committed by Ukrainian nationalists against ethnic Poles during WWII, a spokesman for the Polish Foreign Ministry, Lukasz Jasina, has said.

In an interview with Poland’s Onet online news media outlet on Friday, the official was asked if Zelensky should apologize for the massacre.

“The Ukrainian side did not do this but president Zelensky should take more responsibility [for] Ukraine,” Jasina said. The issue has such an importance for Warsaw that it must be dealt with at the highest level, he added.

The dark legacy of the Volyn massacre still haunts the bilateral relations between the two neighbors and “blocks many joint initiatives,” the ministry’s spokesman said. In 2016, the Polish parliament recognized the Volyn Massacres as genocide at the hands of the Ukrainian Insurgent Army (UPA).

“You can't change that fact. You can't turn back time and there will never be a good time to talk about it…” Jasina said.

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According to the official, saying “I'm sorry and please forgive me” always works very well in Polish-Ukrainian relations “but there is still not enough of it.”

The Volyn massacre was part of an ethnic cleansing campaign waged by UPA nationalist militants against the local Polish population in the regions of Western Ukraine and eastern Poland also known as Volhynia and Galicia between 1943 and 1944, during the Nazi occupation.

According to various estimates, from 40,000 to 60,000 Poles were killed in total.

The UPA was a paramilitary wing of the Organization of Ukrainian Nationalists (OUN) known for its radical anti-Semitic ideology. The UPA assisted Nazi Germany’s troops in the extermination of Jews on the territory of Ukraine on multiple occasions, in addition to the tens of thousands of Poles it killed in western Ukraine.

Glorification of Ukrainian nationalists by Kiev, including the OUN leader, Stepan Bandera, has long been a contentious issue in relations between Ukraine and Poland as well as Israel. In 2018, Polish President Andrzej Duda signed a bill that banned the promotion of the ideology associated with Stepan Bandera. Israel also repeatedly urged Kiev to refrain from glorifying “war criminals.”

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Kiev still apparently fails to understand just how important this issue is for the Poles, Jasina said, adding that “this is… not the most important thing for Ukrainians” and their “understanding is still too weak.”

The people in Poland, however, “clearly said this is more important than some behind-the-scenes diplomatic negotiations,” he added.

Kiev blasted Warsaw over what it called bossing Ukraine around. “Any attempt to impose on the Ukrainian president or Ukraine [and tell us] what we must [do] about our common past is unacceptable and unfortunate,” Ukraine’s ambassador to Warsaw, Vasily Zvarych said on Twitter on Saturday.

He also said that Ukraine “remembers history” and called for what he called “balance and respect in statements.”

Bandera has been officially hailed as a national hero in Ukraine since 2010.

Ukrainian nationalists have regularly marked his birthday on January 1 with torchlit marches and massive demonstrations.

Warsaw has repeatedly protested such actions.

In January, Polish Prime Minister Mateusz Morawiecki vowed to raise the issue of Bandera with Kiev at the first opportunity.