The presidents of Ukraine and Poland met on Sunday to mark the anniversary of a series of massacres of Poles by Ukrainian nationalists during the Second World War which have for generations strained ties between two countries that are now allies.
Poland has positioned itself as one of Kyiv's staunchest supporters since Russia invaded Ukraine last year.
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 in a series of killings that spanned from 1943 to 1945.
Warsaw says around 100,000 Poles were killed in the massacres. Thousands of Ukrainians died in reprisal killings.
Ukraine's Volodymyr Zelenskyy and his Polish counterpart Andrzej Duda both attended a remembrance service for the victims in the western Ukrainian city of Lutsk.
"Together we pay tribute to all the innocent victims of Volhynia! Memory unites us!" Mr Duda's office and Mr Zelenskyy 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".
Mr Duda called the service "a testimony of friendship in the face of a difficult history".
In a post on Twitter, Mr Zelenskyy said he had a "brief but very substantive" discussion at the event with Mr Duda about the upcoming Nato summit in Vilnius, where Ukraine is hoping to see decisions made that will hasten its goal of joining the alliance.
"We agreed to work together to get the best possible result for Ukraine," he wrote.
The head of Mr 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 the historical truth to our Ukrainian friends, it will of course be continued," Pawel Szrot told private broadcaster Polsat News.
The Polish parliament has said that the murders, carried out by the Ukrainian Insurgent Army and the Organisation 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 whether Polish specialists can search for and exhume the remains of Poles who died in Ukraine.
The tensions led to an unusually public row between Poland and Ukraine earlier this year when a Polish Foreign Ministry spokesman said that Mr Zelenskyy 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.