Pope Francis has made one of his strongest pleas yet to end the war in Ukraine, appealing directly to the presidents of Russia and Ukraine on Sunday to find a way out of a crisis that risks becoming a greater threat as top officials in Moscow obliquely threaten the use of nuclear weapons.

Speaking to thousands of faithful gathered in St. Peter’s Square for the weekly Angelus prayer, Francis appealed “first of all” to President Vladimir V. Putin of Russia to stop “this spiral of violence and death,” also “for the sake of his own people.” It was the first time the pontiff has appealed directly to Mr. Putin, one month after the Vatican labeled Russia as the aggressor in the war.

And with Mr. Putin and some top Russian officials hinting at the use of tactical nuclear weapons to defend Ukrainian territory that the Kremlin has sought to annex illegally, the pope said that it was “absurd” that “mankind is once again facing the atomic threat.”

“I strongly deplore the grave situation created in recent days, with further actions contrary to the principles of international law,” he said, referring to Mr. Putin’s annexation of four Ukrainian regions. “Indeed, it increases the risk of nuclear escalation to the point of fearing uncontrollable and catastrophic worldwide consequences.”

The pope defended the right of all countries to their “sovereignty and territorial integrity.” And he expressed his concern for the growing number of victims, especially children, and renewed his calls for an immediate cease-fire and peace negotiations.

Acknowledging the “tremendous suffering” of the Ukrainian population, he urged President Volodymyr Zelensky of Ukraine to be “open to serious proposals for peace.” And he called on world leaders to do everything in their power to end the war and promote talks.

“How much more blood must flow before we understand that war is never a solution but only destruction?” Francis said.

— Elisabetta Povoledo