Pope Francis made the remarks on October 2 in his weekly public prayer on St. Peter's Square at the Vatican.
Pope Francis has called on Russian President Vladimir Putin to stop "this spiral of violence and death" over the war in Ukraine.
Francis's remarks, made on October 2 in his weekly public prayer on St. Peter's Square at the Vatican, were some of the strongest remarks he has made since the February 24 invasion.
"My appeal is addressed first of all to the president of the Russian Federation, begging him to stop, also for the love of his people, this spiral of violence and death," Francis said.
"On the other hand, saddened by the immense suffering of the Ukrainian population following the aggression suffered, I direct an equally confident appeal to the president of Ukraine to be open to serious peace proposals," he said.
The Roman Catholic leader also decried the growing risk of nuclear war, calling it "absurd."
"I deeply regret the serious situation that has arisen in recent days, with further actions contrary to the principles of international law," he said. "In fact, it increases the risk of a nuclear escalation, to the point of fearing uncontrollable and catastrophic consequences worldwide."
The pope's comments came two days after Putin gave a fiery Kremlin speech in which he announced Russia was annexing four regions of Ukraine that are partially occupied by Russian forces.
In the September 30 speech, Putin also made veiled threats about using nuclear weapons in the conflict, echoing earlier remarks in which he warned the West "this is not a bluff."
The Kremlin had no immediate reaction to the pope's comments.