Pope Francis on Wednesday compared the war in Ukraine to a Nazi operation that killed some two million people, mostly Jews, in the first years of World War Two.
Speaking to Polish pilgrims at his weekly general audience, Francis noted that the Catholic University of Lublin, in Poland, had recently commemorated the anniversary of Operation Reinhard.
It was the code name for a secret operation in a part of occupied Poland which the Germans called the "General Government" area, that included territory now in Ukraine.
"May the memory of this horrible event arouse intentions and actions of peace in everyone," he said, specifically mentioning the operation, saying it was one of "extermination".
He then went off script to add: "And history is repeating itself. We see now what is happening in Ukraine."
Russia calls its invasion of Ukraine a "special military operation" to "de-Nazify" the country and root out nationalists it considers dangerous. Ukraine and its allies accuse Moscow of an unprovoked war to grab territory from its pro-Western neighbour.
Since the invasion began in February, Francis has been increasingly forceful in his condemnation of Russia's actions.
Last month, he said Ukrainians were suffering a "martyrdom of aggression" and compared the effects of the war on Ukrainians to the "terrible genocide" of the 1930s, when Soviet leader Josef Stalin inflicted famine on the country.
Francis has several times offered the Vatican's mediation to resolve the conflict but his increased criticism of Russia has made this highly unlikely.
In an interview last month with the Jesuit magazine America, Francis spoke of what he called the cruelty of Russian forces in Ukraine.
"Generally, the cruellest are perhaps those who are of Russia but are not of the Russian tradition, such as the Chechens, the Buryati and so on. Certainly, the one who invades is the Russian state. This is very clear," said Francis.
Russian Foreign Minister Sergei Lavrov called the comments "un-Christian" and incomprehensible.
(Reporting by Philip Pullella; Editing by Crispian Balmer)
