The leader of Poland's ruling conservative party has commemorated a massacre of Poles by Ukrainians during World War II, describing it as genocide.

Jaroslaw Kaczynski, head of the Law and Justice party, laid flowers Monday at a monument in Warsaw to the victims of the Volyn massacre on the 73th anniversary of a key moment in the killings.

Kaczynski said "we must never let this crime against Poles and any such crime be overlooked, relativized or described as anything but genocide."

From 1943-1944, Ukrainian nationalists killed up to 100,000 Poles in Volyn and eastern Galica, areas then in Poland but now in Ukraine. The peak of the killings, which involved Poles being butchered with axes and saws, was on July 11, 1943.

About 20,000 Ukrainians died at Polish hands.