Kyiv is working with allies on security guarantees akin to NATO's mutual protection pledge, Ukraine's president Volodymr Zelensky has said. Officials from Ukraine and allies the U.S., the U.K., France and Germany met in London on Thursday in a week when a peace proposal was produced which outlined an Article 5-like agreement which would be backed by Washington.

Trump on Saturday criticized Russia's Vladimir Putin for attacks on civilian areas of Ukraine in recent days, and said "maybe he doesn't want to stop the war."

US President Donald Trump cast doubt on whether Russian leader Vladimir Putin wants a peace deal, shortly after meeting Ukrainian President Volodymyr Zelensky in the hush of St Peter's Basilica before Pope Francis's funeral Saturday.