Trump made the pledge during an extraordinary summit at the White House on Monday, where he hosted Zelenskyy and a group of European allies after Trump met Russian President Vladimir Putin in Alaska on Friday.
"When it comes to security, there's going to be a lot of help," Trump told reporters, adding that European countries would be involved. "They are a first line of defence because they're there, but we'll help them out."
Zelenskyy hailed the promise as "a major step forward," adding that the guarantees would be "formalized on paper within the next week to 10 days" and saying Ukraine offered to buy about $US90 billion ($NZ151 billion) worth of US weapons.
The tone on Monday was much warmer than a disastrous Oval Office meeting that saw Trump and Vice President JD Vance publicly criticise the Ukrainian leader in February.
But a peace deal still appeared far from imminent.
Just before the talks began, Russia's Foreign Ministry ruled out the deployment of troops from Nato countries to help secure a peace deal, adding complications to Trump's offer.
Both Trump and Zelenskyy said they hoped Monday's gathering would eventually lead to three-way talks with Putin. The Kremlin has not yet publicly accepted such a meeting, and it remains to be seen whether Putin, whose forces have been slowly grinding forward in eastern Ukraine, is prepared to sit down with Zelenskyy or make meaningful concessions.
In a social media post late on Monday, Trump said he had called Putin and begun arranging a meeting between Putin and Zelenskyy, to be followed by a trilateral summit among the three presidents. A source from the European delegation said Trump had told European leaders that Putin suggested he meet Zelenskyy directly first.
The last direct talks between Russia and Ukraine took place in Turkey in June. Putin declined Zelenskyy's public invitation to meet him face-to-face there and sent a low-level delegation instead.
Kremlin aide Yuri Ushakov said in audio remarks on Telegram on Monday that Trump and Putin had discussed "the possibility of raising the level of representatives from the Ukrainian and Russian sides - that is, those representatives participating in the mentioned direct negotiations."
Meanwhile, European leaders - who rushed to Washington in support of Ukraine - urged Trump to insist that Putin agree to a ceasefire in the three and a-half-year old war before any talks can advance. Trump previously backed that proposal but reversed course after meeting with Putin on Friday, instead adopting Moscow's position that any peace agreement be comprehensive.
Trump told reporters in the Oval Office that he liked the concept of a ceasefire but the two sides could work on a peace deal while the fighting continued.
"I wish they could stop, I'd like them to stop," he said. "But strategically that could be a disadvantage for one side or the other."
Putin's special envoy for investment and economic cooperation, Kirill Dmitriev, in an X post called it "an important day of diplomacy today with the focus on Lasting Peace not a Temporary Ceasefire."
Later in the day, German Chancellor Friedrich Merz and French President Emmanuel Macron both voiced support for a ceasefire as a prerequisite to any direct talks with Russia.
"I can't imagine that the next meeting would take place without a ceasefire," said Merz, seated with Trump and other leaders. "So let's work on that and let's try to put pressure on Russia."
Macron also said European leaders would eventually need to be included in any peace talks.
"When we speak about security guarantees, we speak about the whole security of the European continent," he told Trump.
Trump and Zelenskyy spoke in private before joining the rest of the European leaders for more than two hours of multilateral talks.
FRIENDLY TONE AFTER FEBRUARY DISASTER
Zelenskyy navigated Monday's meeting much more successfully than his Oval Office encounter in February, which ended abruptly when Trump and Vance publicly upbraided Zelenskyy as not being grateful enough.
In his opening remarks to the media on Monday, Zelenskiy repeated his thanks at least eight times, striking a deferential tone.
Rather than visiting alone, Zelenskyy had reinforcements this time. The European leaders travelled to Washington to demonstrate solidarity with Kyiv and push for strong security guarantees for the country in any post-war settlement.
Trump greeted Zelenskyy warmly upon his arrival outside the White House, expressing admiration for his black suit. That was a departure from the Ukrainian leader's typical military clothes, which media reports said irritated Trump in February.
When a reporter asked Trump what his message was to the people of Ukraine, he said, "We love them." Zelenskyy thanked him, and Trump put his hand on Zelenskyy's back in a show of affection before the two men went inside to the Oval Office.
Zelenskyy described his direct talks with Trump as "very good" and said they had spoken about Ukraine's need for US security guarantees.
"This is very important, that the United States gives such a strong signal and is ready for security guarantees," Zelenskyy said.
Trump said European countries would be "a first line of defence because they're there, but we'll help them out."
TRUMP UPS PRESSURE
Trump has pressed for a quick end to Europe's deadliest war in 80 years, and Kyiv and its allies worry he could seek to force an agreement on Russia's terms after the president on Friday rolled out the red carpet - literally - for Putin, who faces war crimes charges from the International Criminal Court.
Officials in Ukraine said Russian attacks overnight on Ukrainian cities killed at least 10 people, including a toddler, in what Zelenskyy called a "cynical" effort to undermine talks.
Trump has rejected claims that the Alaska summit was a win for Putin, who has faced diplomatic isolation since Russia's invasion of Ukraine in 2022.
"I know exactly what I'm doing, and I don't need the advice of people who have been working on all of these conflicts for years, and were never able to do a thing to stop them," Trump wrote on social media.
Trump's team has said both sides must compromise. But the president has put the burden on Zelenskyy, saying Ukraine should give up hopes of regaining Crimea, annexed by Russia in 2014, or of joining the Nato military alliance.
Zelenskyy has already all but rejected the outline of Putin's proposals from the Alaska meeting. Those include handing over the remaining quarter of its eastern Donetsk region, which is largely controlled by Russia.
Any concession of Ukrainian territory would have to be approved by a referendum.
The war has killed or wounded more than a million people from both sides, including thousands of mostly Ukrainian civilians, according to analysts, and destroyed wide swaths of the country.
