Pope Francis, ramping up his implicit criticism of Russia, on Friday called the war in Ukraine a "perverse abuse of power" waged for partisan interests which has condemned defenseless people to violence.

The pope has not actually named Russia in his condemnations but he has used phrases such as "unacceptable armed aggression" to get his point across and on Friday spoke of "people defending their land" and escaping bombardments.

"The tragedy of the war taking place in the heart of Europe has left us stunned," he said, adding that few people would have imagined scenes similar to the two world wars in the 20th century.

His latest condemnation came in a message to a Catholic Church conference in Bratislava, the capital of Slovakia, one of the countries bordering Ukraine that has opened its doors to refugees.

"Once more humanity is threatened by a perverse abuse of power and partisan interests which condemns defenseless people to suffer every form of brutal violence," he said.

"The blood and tears of children, the suffering of women and men who are defending their land or fleeing from bombardments shakes our conscience," he said.

Moscow says its action is a "special military operation" designed not to occupy territory but to demilitarize and "de-Nazify" its neighbor.

The pope has rejected that term, however, saying previously it could not be considered "just a military operation" but a war that had unleashed "rivers of blood and tears."

On Wednesday held a video call with Patriarch Kirill of the Russian Orthodox Church, an ally of Russian President Vladimir Putin.

Kirill, 75, has made statements defending Moscow's actions in Ukraine and sees the war as a bulwark against a West he considers decadent, particularly over the acceptance of homosexuality.

The Vatican said the pope told Kirill: "The ones who pay the price of war are the people, the Russian soldiers and the people who are bombarded and die."