Trump had shown willingness to join the guarantees at a last-ditch virtual meeting with European leaders and Ukrainian President Volodymyr Zelenskiy on Wednesday, leaders said, though he made no public mention of them afterwards.
Zelenskiy and his allies have voiced some optimism as they intensified efforts to prevent any deal between Trump and Russia's Vladimir Putin at a meeting in Alaska on Friday that would leave Ukraine vulnerable to further Russian attacks.
Friday's summit comes at one of the toughest moments for Ukraine in a war, the largest in Europe since World War II, that has killed tens of thousands and displaced millions since Russia's full-scale invasion in February 2022.
Speaking after Wednesday's meeting, French President Emmanuel Macron said Trump insisted that the transatlantic NATO alliance should not be part of security guarantees that would be designed to protect Ukraine from future attacks in a post-war settlement.
"President Trump also stated this clearly, saying things that I find important: namely, that NATO should not be part of these security guarantees - and we know this is a key point, particularly for the Russian side - but (also) that the United States and all willing allies should be part of them. That is what we are committed to," Macron said.
"And for me, this was an important clarification today."
German Chancellor Friedrich Merz, who hosted Wednesday's meeting, also said there would be robust security guarantees. "
"President Trump also confirmed this today and said he is on board," he told reporters.
A European Commission spokesperson also welcomed Trump's offer but said the details were up to the White House to answer.
On Wednesday, Trump threatened "severe consequences" if Putin does not agree to peace in Ukraine and while he did not specify what the consequences could be, he has warned of economic sanctions if his meeting on Friday proves fruitless.
However, Russia is likely to resist Ukraine and Europe's demands strongly and previously has said its stance had not changed since it was first detailed by Putin in June 2024.
To gear up for the Alaska summit, Putin held a meeting with top officials and representatives of Russia's leadership, the TASS state news agency reported, citing Kremlin spokesman Dmitry Peskov.
A Kremlin aide said Putin and Trump will discuss the "huge untapped potential" for Russia-US economic ties as well as the prospects for ending the war at the meeting, the first summit between their countries since Putin met Joe Biden in 2021.
Zelenskiy confirmed this week that Russian forces had advanced by about 10km near the town of Dobropillia in the Donetsk region. Ukraine, suffering manpower challenges, was forced to move in reserves to stabilise the situation.
Trump described the aim of his talks with Putin in Alaska as "setting the table" for a quick follow-up that would include Zelenskiy.
Trump has said a deal could include what he called a land swap. Russia controls around a fifth of Ukraine and a land swap within Ukraine could cement Moscow's gains.
Zelenskiy and the Europeans worry that would reward Putin for nearly 11 years of efforts to seize Ukrainian land and embolden him to expand further west in Europe.
Trump's agreement last week to the summit was an abrupt shift after weeks of voicing frustration with Putin for resisting the US peace initiative.
As conditions for a ceasefire and the start of talks, Putin has demanded Ukraine withdraw its forces from four regions that Russia has claimed as its own but does not fully control, and formally renounce plans to join NATO.
Kyiv swiftly rejected the conditions as tantamount to surrender.
