The Vatican sought Tuesday to defuse a row with Ukraine after Kyiv accused Pope Francis of spreading "imperial propaganda" in a video message to young Catholic Russians.

The pope told the group gathered in a Catholic church in St Petersburg that "you are children of great Russia, of great saints, of kings, of Peter the Great, of Catherine II, of a Russian people of great culture and great humanity", according to a video posted online.

"Never forget this great legacy. You are heirs of the great Mother Russia, go forward with that."

Ukrainian foreign ministry spokesman Oleg Nikolenko said the pope's language was "very unfortunate".

"It is with this kind of imperialist propaganda, 'spiritual staples' and the 'need' to save 'great Mother Russia', that the Kremlin justifies the murder of thousands of Ukrainians and the destruction of hundreds of Ukrainian cities and villages," he said late Monday.

"It is very unfortunate that Russian great-power ideas, which are, in fact, the cause of Russia's chronic aggressiveness, are consciously or unconsciously coming from the mouth of the pope, whose mission, in our understanding, is precisely to open the eyes of Russian youth to the destructive course of the current Russian leadership."

Vatican spokesman Matteo Bruni issued a statement Tuesday saying the pontiff's "off the cuff" remarks were intended to "encourage young people to preserve and promote what is positive in Russia's great cultural and spiritual heritage".

The comments were "certainly not (intended) to glorify imperial logic and government personalities".

Pope Francis regularly calls for peace in Ukraine, though in the early months after Russia's February 2022 invasion he drew criticism for not naming Moscow as the aggressor.

He appointed a top cardinal earlier this year to try to broker peace, who has since visited both Moscow and Kyiv.

The Vatican's official media portal reported on the pope's video message to mark Russian Youth Day on August 25, but did not include a video or the specific quotes Kyiv objected to.

Instead, it reported the pope calling on the Russian youths to be "artisans of peace" and to "sow seeds of reconciliation".

The Kremlin, for its part, welcomed the pope's remarks.

"The pontiff knows Russian history. That's very good," government spokesman Dmitry Peskov said.