Volodymyr Zelenskyy has called for a "clear position" from Donald Trump to stop Vladimir Putin and end the war in Ukraine.
In an exclusive interview with Sky News' lead world presenter Yalda Hakim, the Ukrainian president said the only way for the fighting to stop was for defined security guarantees to first be put in place.
And that, he said, could only come if Mr Trump was bold.
He told Sky News he hopes UK Prime Minister Sir Keir Starmer would drill into the detail of securing Ukraine's future with the president during his state visit to Britain this week.
He said: "I very much hope he (Starmer) will be able to have a very specific discussion on the security guarantees of the US for Ukraine.
"Before we end the war, I really want to have all the agreements in place. I want to… have a document that is supported by the US and all European partners. This is very important.
"To make this happen, we need a clear position of President Trump."
'Make Putin afraid'
Mr Zelenskyy also urged the US leader to take "strong personal steps" to "stop Putin", after Mr Trump urged NATO allies to stop buying Russian oil and put tariffs on China to pressure Moscow.
"I believe that the US is strong enough to take decisions of their own," he said. "I believe Donald Trump can give us air defence systems in quantity and US has enough.
"I'm sure the US can apply enough sanctions in order to hurt the Russian economy, plus Donald Trump has enough force to make Putin afraid of him.
"Europe has already introduced 18 sanctions packages against Russia. And all that's lacking now is a strong sanctions package from the US."
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His comments came following criticism in his interview with Sky News - at the Presidential Palace in Kyiv - of the recent Trump-Putin summit in Alaska.
He said Mr Trump "gave a lot to Putin" and that "he should have paid more" for it.
"I believe, if it was a trilateral meeting [with Ukraine included], we would have some result," he added.
Putin 'testing NATO', warns Zelenskyy
As news broke that British fighter jets were flying air defence missions over Poland after a Russian drone incursion, Hakim asked the Ukrainian leader what message he thought Putin was sending to Europeans.
"He's testing NATO," he said. "He wants to see what NATO is ready for, what they're capable of, both diplomatically and politically, and how the local population will respond to this."
"Also, in my opinion, the other message they are sending is, 'don't you dare to give Ukraine additional air defence systems, because you might need them yourself.'"
Mr Trump arrives in the UK today for an unprecedented second state visit, following an invitation from King Charles.
He and First Lady Melania will stay at Windsor Castle and be treated to a flypast by the Red Arrows as well as UK and US F-35 military jets on the east lawn, and a special Beating Retreat military ceremony.
They will also visit Chequers, the prime minister's official country residence in Buckinghamshire, though details of what they will discuss - and whether it will include the situation in Ukraine - have not been revealed.
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