Ukrainian President Volodymyr Zelenskyy said he discussed the weekend's turmoil in Russia in phone calls with the leaders of the United States, Canada, and Poland on Sunday, and that the "weakness" of Kremlin leader Vladimir Putin had been "exposed."

The phone calls took place after an extraordinary failed mutiny by Russian mercenary boss Yevgeny Prigozhin on Saturday that raised questions about Putin's grip on power as Ukraine presses a counteroffensive in its south and east.

"We discussed the course of hostilities and the processes taking place in Russia. The world must put pressure on Russia until international order is restored," Zelenskyy said after a phone call with U.S. President Joe Biden.

He said he and Biden had also discussed further expanding defense cooperation with an emphasis on long-range weapons, coordination ahead of the NATO summit in Vilnius next month, and preparations for a "Global Peace Summit" he has promoted.

"Yesterday's events exposed the weakness of Putin's regime," the statement said.

In another similar statement, Zelenskyy said he had told Canadian Prime Minister Justin Trudeau in a phone call about the "threatening situation" at Ukraine's vast, Russian-occupied Zaporizhzhia nuclear power plant.

Zelenskyy warned earlier this week that Russia was considering carrying out an act of "terrorism" involving the release of radiation at the plant, an allegation denied by Russia.

"Ukraine's partners must demonstrate a principled response, in particular at the NATO Summit in Vilnius," he said.

The Ukrainian leader made similar comments in a statement announcing a phone call with Polish President Andrzej Duda.