Donald Trump said Volodymyr Zelensky could end the war with Russia "almost immediately" as the Ukrainian president and European leaders including Sir Keir Starmer prepared for crunch White House meetings.
The US president suggested Mr Zelensky would have to accept there was "no getting back" Crimea, which was annexed by Russia in 2014, and Ukraine would not be allowed to join the Nato alliance.
Sir Keir and other European leaders will seek to persuade Mr Trump not to push for a settlement which rewards Vladimir Putin's aggression and to get US security guarantees for any military peacekeeping force from the so-called "coalition of the willing".
Those joining Sir Keir include France's Emmanuel Macron, Germany's Friedrich Merz, Italy's Giorgia Meloni and Alexander Stubb, president of Finland.
Nato chief Mark Rutte and European Commission president Ursula von der Leyen are also attending.
In a message on his Truth Social platform, Mr Trump said President Zelensky "can end the war with Russia almost immediately, if he wants to, or he can continue to fight".
He said there would be "no going into Nato by Ukraine" - keeping its neighbour out of the alliance and its mutual defence pact has been one of Russia's key aims.
But Sir Keir, along with other Nato leaders, has said Ukraine is on an "irreversible path" to membership of the alliance.
Government minister Stephen Kinnock said the "pathway for Ukraine to Nato" could not be dictated by any other country.
The health minister told Times Radio: "Any decisions taken about Ukrainian territory must be taken with the agreement of the Ukrainian government and President Zelensky.
"The other is that the pathway for Ukraine to Nato and to security guarantees cannot be dictated to them by any other country, and the other is to send a very clear message that we the British people stand firmly shoulder-to-shoulder with the Ukrainian people as we showed when we opened our homes and our hearts to the Ukrainian refugees."
Mr Trump's envoy Steve Witkoff has suggested that measures similar to Nato's Article 5 mutual defence provision could be offered by the US without Kyiv joining the alliance.
Mr Witkoff, who took part in the talks between Mr Trump and Russian president Mr Putin last week, said it "was the first time we had ever heard the Russians agree to that" and called it "game-changing".
"We were able to win the following concession: That the United States could offer Article 5-like protection, which is one of the real reasons why Ukraine wants to be in Nato," Mr Witkoff told CNN.
Mr Zelensky said any peace deal must be lasting "not like it was years ago, when Ukraine was forced to give up Crimea and part of our East - part of Donbas - and Putin simply used it as a springboard for a new attack".
He said: "Russia must end this war, which it itself started. And I hope that our joint strength with America, with our European friends, will force Russia into a real peace."
At the White House, Mr Zelensky is expecting to face calls from the US president to concede to full Russian control of Donetsk and Luhansk, two mineral-rich regions of Ukraine that are mostly occupied by Vladimir Putin's forces.
In exchange for these demands, the Russian president would reportedly withdraw his forces from other areas of Ukraine and accept the Nato-like guarantee designed to prevent him launching further incursions.
Ahead of their Oval Office encounter, the allies are likely to be mindful of the previous occasion Mr Zelensky visited Mr Trump in the White House.
February's public spat, which saw Vice President JD Vance accuse Mr Zelensky of not being thankful enough to the US, resulted in American aid to Ukraine being temporarily halted.
Mr Trump will again host Mr Zelensky in the Oval Office before a separate meeting with the European leaders.