Pope Francis on Wednesday condemned Russia's "relentless bombings" of Ukrainian cities, saying the attacks had unleashed a "hurricane of violence" on residents.

Speaking to thousands of people at his weekly general audience in St. Peter's Square, he also appealed to "those who have the fate of the war in their hands" to stop.

At least 26 people have been killed across Ukraine during Russia's biggest aerial offensive since the start of its invasion in February.

"My heart is always with the Ukrainian people, especially the residents of the places that have been hit by relentless bombings," Francis said.

"May (God's) spirit transform the hearts of those who have the fate of the war in their hands, so that the hurricane of violence stops and peaceful coexistence in justice can be rebuilt."

In Amsterdam, prosecutors for International Mobile Justice teams are investigating as possible war crimes the ongoing Russian missile strikes on Kyiv and other cities.

Asked on the sidelines of an event in Rome if the attacks could be considered war crimes, the Vatican's number two, Secretary of State Cardinal Pietro Parolin, said he was not qualified to make such a technical, juridical judgment.

"But certainly they are unacceptable acts that cry out vengeance before God and before humanity because bombing unarmed civilians is beyond any logic. It is to be totally condemned," Parolin said.

Monday's attacks killed 19 people, wounded more than 100 and knocked out power across the country, Ukrainian officials said. More strikes on Tuesday killed seven people in the southeastern town of Zaporizhzhia and left part of the western city of Lviv without power.

Russia denies targeting civilians in its military operation in Ukraine, and has accused the West of escalating and prolonging the conflict by supporting Kyiv.

Francis directly called on Russian President Vladimir Putin for the first time 10 days ago to stop the "spiral of violence and death", saying the crisis was risking uncontrollable global consequences.