Pope Francis has compared the war in Ukraine to the Nazi's Operation Reinhard.
The plans saw the death of two million people - mostly Jews - in the early days of the Second World War.
The comparison was made when visiting a Catholic university in Poland that had recently commemorated the tragedy.
Operation Reinhard was executed in Poland during the occupation. Some of the deaths occurred in areas that now belong to Ukraine.
"May the memory of this horrible event arouse intentions and actions of peace in everyone," he said.
"And history is repeating itself. We see now what is happening in Ukraine," he continued.
Pope Francis has been persistent in his calls for peace in the eastern European nation, which has been at war with Russia for ten months.
The pontiff has made multiple offers to act as a mediator between the warring countries in a bid to seek a resolution.
Recently, the Kremlin said they were open to discussions with the pope, but that Ukraine was making it impossible.
Many of the global leaders offering to help resolve the conflict refuse to recognise the results of votes held in Russian-occupied areas of Ukraine, because they do not believe the results of the so-called elections are correct.