The Ukrainian and Polish presidents jointly marked the anniversary on Sunday of World War Two-era massacres of Poles by Ukrainian nationalists, killings that have caused tension for generations between countries that are now close allies.
Warsaw has positioned itself as one of Kyiv's staunchest supporters since Russia invaded the country in 2022.
But the Volhynia massacres have continued to hang over ties between the two nations, particularly ahead of the July 11 anniversary of one of the bloodiest days of a series of killings that took place from 1943 to 1945.
Poland says around 100,000 Poles were killed in the massacres by Ukrainian nationalists. Thousands of Ukrainians also died in reprisal killings.
Ukraine's Volodymyr Zelenskiy and his Polish counterpart Andrzej Duda attended a church service together in the western Ukrainian city of Lutsk, in memory of the victims.
"Together we pay tribute to all the innocent victims of Volhynia! Memory unites us!," Duda's office and Zelenskiy both wrote on Twitter. "Together we are stronger."
The service was attended by the heads of the largest Orthodox and Catholic churches in Ukraine and the head of the Polish Bishop's Conference, Archbishop Stanislaw Gadecki.
The Ukrainian president's chief of staff, Andriy Yermak, wrote on Telegram that Ukraine and Poland were "united against a common enemy who dreamed of dividing us".
Duda called the service "a testimony of friendship in the face of a difficult history".
In a post on Twitter, Zelenskiy said he had a "brief but very substantive" discussion at the event with Duda about the upcoming NATO summit in Vilnius, where Ukraine is hoping for decisions that will hasten its goal of membership in the alliance.
"We agreed to work together to get the best possible result for Ukraine," Zelenskiy wrote.
The head of Duda's office said the fact that the presidents were commemorating the victims together was "historic", but that more work was needed.
"This is not the end of this difficult road, explaining to our Ukrainian friends about the historical truth, it will of course be continued," Paweł Szrot told private broadcaster Polsat News.
The Polish parliament has said that the murders, carried out between 1943 and 1945 by the Ukrainian Insurgent Army and the Organization of Ukrainian Nationalists under the leadership of Stepan Bandera, bore elements of genocide.
Ukraine has not accepted that assertion and often refers to the Volhynia events as part of a conflict between Poland and Ukraine which affected both nations.
Warsaw and Kyiv have also clashed over the issue of whether Polish specialists can search for and exhume the remains of Poles that died in Ukraine.
The massacre caused an unusually public row between Poland and Ukraine earlier this year, after a Polish foreign ministry spokesman said that Zelenskiy should apologise and ask for forgiveness for the events in Volhynia.
However, Ukraine's parliament speaker Ruslan Stefanchuk moved to defuse tensions in May when he told the Polish parliament that Kyiv understood Poland's pain.
Źródło: TVN24 News in English, Reuters