Volodymyr Zelenskyy has told Sky News that Vladimir Putin could attack a NATO country within five years to test the alliance.
The Ukrainian president made the comments in an interview with chief presenter Mark Austin.
But when asked if Russia could attack within months, Mr Zelenskyy said he did not "believe [Putin] is ready".
Mr Zelenskyy also said plans for NATO members to increase defence spending to 5% of GDP by 2035 are "very slow" - adding: "We believe that, starting from 2030, Putin can have significantly greater capabilities.
"Today, Ukraine is holding him up, he has no time to drill the army."
Russia's soldiers are "all getting annihilated and wiped out at the battlefield", he warned.
"In any case, [Putin] needs a pause, he needs sanctions to be lifted, he needs a drilled army.
"And 10 years is a very long time. He will have a new army ready [by then]."
In a wide-ranging interview, the Ukrainian president said:
• Tensions in the Middle East may mean Ukraine gets less aid from partners including the US - meaning "Russia will feel the advantage on the battlefield"
• British missile components are making their way into Russia
• He may not be the best man to lead Ukraine through the rest of the war, but he is "the most experienced"
• The US and Russia "may be short-term partners, but they will never be friends" while Donald Trump is in power
• Mr Zelenskyy also refused to answer whether he felt "bullied" during their disastrous Oval Office meeting - but said he "conducted himself honestly"
Asked about his views on the Israel-Iran conflict, and the impact of a wider Middle East war for Ukraine, Mr Zelenskyy said he understood that aid from partners, including the US "may be reduced", adding "it will be difficult for us".
On his dealings with Donald Trump, Mr Zelenskyy avoided any criticism and focused instead on how the US president interacts with Mr Putin, calling them "short-term partners" but not friends.
On the topic of a potential ceasefire and peace talks, Mr Zelenskyy was clear he supported both, adding that he would enter negotiations to understand "if real compromises are possible and if there is a real way to end the war".
But he avoided saying whether he would be willing to surrender land to Russia as part of any peace deal, instead suggesting military force and sanctions would force Mr Putin into negotiations.
The Ukrainian leader also spoke about the failure of international sanctions. He said Russia was still able to access "components for missiles and drones" from countries, including the UK.
The UK and its NATO allies will formally sign off the defence spending plans when the heads of state and government meet in The Hague on Tuesday and Wednesday.
The spending goal is broken down into 3.5% of GDP to be spent on pure defence and 1.5% of GDP on related areas, such as infrastructure and cybersecurity.
Defence spending of 5% is the kind of level invested by NATO allies during the Cold War.
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The Ukrainian president, who has been invited to the NATO summit, admitted that Ukraine's ambition to join the military alliance "isn't possible right now" but said that over the longer term "NATO needs Ukrainians".
Mr Zelenskyy met with Sir Keir Starmer at Downing Street and Commons Speaker Sir Lindsay Hoyle at parliament on Monday, before travelling to Windsor Castle for a meeting with the King.
On those discussions, the Ukrainian president said: "I have always had very good dialogue with him [Charles]" and described him as "a bright man".
(c) Sky News 2025: Russia could attack a NATO member 'within five years', Volodymyr Zelenskyy warns
