German President Frank-Walter Steinmeier on Sunday asked Poland's forgiveness for history's bloodiest conflict during a ceremony in the Polish city of Wielun, where the first World War II bombs fell 80 years ago.

"I bow before the victims of the attack on Wielun. I bow before the Polish victims of German tyranny. And I ask your forgiveness," Steinmeier said in both German and Polish.

Poland suffered some of the worst horrors of World War II: Nearly 6 million Poles died in the conflict that killed more than 50 million people overall. 

That figure includes the 6 million Jews who died in the Holocaust, half of them Polish.

"It was Germans who committed these crimes against humanity in Poland. Anyone calling them things of the past, or claiming that the vile rule of terror of the National Socialists in Europe was a mere footnote of German history, is passing judgement on him or herself," Steinmeier added in the presence of his Polish counterpart.

The line appeared to be a clear reference to the German far-right, whose co-leader Alexander Gauland once called the 12-year Third Reich a "speck of bird poop" on an otherwise glorious German past.

"As Germany's Federal President, let me assure you that we will not forget," Steinmeier said.

"We want to, and we will, remember. And we will bear the responsibility that our history imposes upon us." 

Polish President Andrzej Duda for his part denounced Nazi Germany's attack on Poland, calling it "an act of barbarity" and "a war crime."

"I am convinced that this ceremony will go down in the history of Polish-German friendship," he added, thanking Steinmeier for his presence.

The heads of state will later tour the Wielun museum and meet with ­local survivors of the September 1, 1939 bombing. 

Although it has been 80 years since the war started, there are still unresolved matters according to Poland, which says Germany owes it war reparations.

A parliamentary commission is currently working on a new analysis of the extent of Poland's wartime human and material losses. 

"We have to talk about, remember and demand the truth regarding those losses. We have to demand compensation," Morawiecki said Sunday at the Westerplatte ceremony.

When it comes to reparations, however, Berlin believes the case is closed.