As Ukraine marks 75 years since the Nazi slaughter of tens of thousands of Jews in Kiev’s Babi Yar ravine, President Reuven Rivlin warned against succumbing to the “sin” of forgetting or denying the atrocities of the Holocaust.

“We must not play a part in the sin of forgetting or denial,” Rivlin told Ukrainian lawmakers at a special parliament session dedicated to commemorating one of the most notorious massacres of WWII.

“National leaders who support anti-Semitic, racist, or neo-Nazi ideas will not be welcomed as friends among the family of nations,” he said.

Rivlin was in Ukraine to participate in memorial events commemorating the 75th anniversary of the murder of more than 33,000 Jews — mostly elderly, women and children — on September 29-30, 1941, as Hitler’s forces advanced toward Moscow on the eastern front.

In his address, Rivlin also urged nations to look towards the future.

“The valley saw two horrific sins. The first sin, was the sin of murder and destruction. The second sin was the sin of concealment and destruction of the memory,” he told lawmakers. “The second sin was no less systematic nor relentless than the first – it was as comprehensive as the massacre itself.”

“The blood of our brothers and sisters, that was spilled at that dark time, places upon us the duty to remember, and teach the whole world, about the dangers of not just anti-Semitism, but of all hatred, and all racism,” Rivlin said in a separate address on Tuesday. “While we mourn the past, we must also speak about the present, and look to the future.”

Rivlin, who also delivered remarks at the residence of Ukrainian President Petro Poroshenko earlier on Tuesday, hailed the 25 years of diplomatic relations between Jerusalem and Kiev and the recent surge in cooperation with the current government.

Bilateral cooperation in the fields of science and technology amounts to a billion dollars in trade annually, the president said, added that Israel looked forward to building on that figure.

Rivlin also expressed support for Ukraine’s deepening economic ties with the West and its ongoing territorial dispute with Russia over the Crimean Peninsula which Russia invaded and annexed in 2014.

“That which we wish for ourselves, we wish for our friends the Ukrainians; freedom, real democracy, and true prosperity,” Rivlin said, adding that “Israel knows that the path upon which Ukraine walks is not easy…and [that] Ukraine faces difficulties and challenges. In this difficult time, we support the Ukrainain government’s courage to enact important reforms in social and economic fields.”

“Israel is helping and will continue to help Ukraine to advance toward a better future,” Rivlin said.