80th anniversary of Volhynian Bloody Sunday. PM: Poland will not rest until all victims are found and laid to rest

Poland has become one of Ukraine's staunchest allies since Russia invaded in 2022, but ties between the neighbours have been strained for generations by the Volhynia killings that took place from 1943 to 1945.

"That crime was so unique that it has to be called by its name ... it was genocide," Mateusz Morawiecki said during a commemoration ceremony in Warsaw.

"There will be no final Polish-Ukrainian reconciliation without finding all the remains, without them finally being honoured."

Poland says over 100,000 Poles were killed in the massacres by Ukrainian nationalists. Thousands of Ukrainians also died in reprisal killings.

The area where the massacres took place, which was inhabited by both Poles and Ukrainians, was part of Poland before World War Two before being occupied by the Soviet Union.

In 2013, the Polish parliament recognised the massacre by the Ukrainian Insurgent Army (UPA) during World War Two as "ethnic cleansing bearing the hallmarks of genocide".

Ukraine has not accepted that assertion and often refers to the Volhynia events as part of a conflict between Poland and Ukraine that affected both nations.

Poland wants Ukraine to allow its specialists unlimited access to sites where the remains of those killed could be buried so they can be exhumed and receive proper funerals.

In 2017, Ukraine banned Polish authorities from searching for victims on its territory. However, in 2022 Kyiv gave permission for Poland to search for victims in one village.

On Sunday, Ukrainian President 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.

Tuesday's commemorations in Warsaw were attended by Ukraine's parliament speaker Ruslan Stefanchuk, who moved to defuse tensions in May when he told the Polish parliament that Kyiv understood Poland's pain.