The high-profile talks came days after Trump met with Russian President Vladimir Putin in Anchorage, Alaska.
One of the main topics on the agenda was security guarantees for Ukraine, broadcaster CNN reported.
European leaders were expected to press Trump to join them in taking a tougher stance with Putin to prevent future conflicts.
Before going behind closed doors, the leaders spoke briefly with reporters.
Trump said discussions would focus on "who will do what" as part of Ukraine’s security guarantees and confirmed that "possible exchanges of territory" to end the war were also on the table, according to CNN.
Zelensky said he had a "very good conversation" with Trump earlier in the day about security and humanitarian issues.
He added he would welcome Trump's participation in a trilateral meeting with Putin.
NATO Secretary-General Mark Rutte called Trump’s willingness to back security guarantees for Ukraine a “breakthrough” toward peace and thanked him for "breaking the deadlock" and bringing Putin to the table.
British Prime Minister Keir Starmer said the "Coalition of the Willing"—Ukraine's core European allies—was "prepared to step up to the plate" to ensure the country’s security.
French President Emmanuel Macron said a European leader should be present at a meeting with Putin.
"When we speak about security guarantees, we speak about the whole security of the European continent," Macron said, according to CNN.
German Chancellor Friedrich Merz said Trump opened a path to more serious negotiations with his meeting with Putin in Alaska last week.
He argued that a ceasefire is needed before more talks are held.
Finnish President Alexander Stubb praised recent progress.
"I think in the past two weeks, we’ve probably had more progress in ending this war than we have in the past three and a half years," he said.
Earlier on Monday, Trump told reporters in the Oval Office he believed momentum was building toward resolving the conflict and suggested his recent meeting with Putin could yield results.
Speaking alongside Zelensky, Trump also said he believed Putin wanted the Ukraine war to end.
He added that he could try to negotiate an end to the conflict without first agreeing on a ceasefire, the Reuters news agency reported.
Trump also said he planned to call Putin after his meeting with Zelensky and that a three-way meeting was possible.
Zelensky said he was open to holding elections in his country once the war ends and conditions are safe.
"We need to work in parliament because during the war you can't have elections," he told reporters, adding that people must be able to vote in a democratic, open and legal election.
During their meeting in the Oval Office, Zelensky also handed the US president a letter from the Ukrainian first lady to Melania Trump.
Ahead of his meeting with Zelensky and European leaders, Trump told Ukraine to give up hopes of regaining Crimea and joining NATO.
Trump and Zelensky previously met at the White House in February, but that meeting descended into a shouting match, as the two leaders engaged into a heated exchange over Russia's war and Washington's support for Kyiv.
During the exchange, Trump and Zelensky, joined by US Vice President JD Vance, raised their voices in an argument over the nature of US support and whether Ukraine had shown sufficient gratitude.
Earlier this year, Trump denounced Zelensky as "a dictator without elections" and warned that he must "move fast" to secure peace with Russia or risk losing his country.
In response, Zelensky said at the time that he was willing to step down if it would guarantee peace in Ukraine, adding that he could trade his resignation for the country's NATO membership.
Zelensky's official five-year term as president expired last year, but he has opposed holding elections while Ukraine remains under full-scale Russian invasion—a stance backed by his major domestic political opponents, according to reports.
Trump in February also mocked Zelensky as "a modestly successful comedian" and accused him of talking the United States into spending USD 350 billion "to go into a war that couldn’t be won, that never had to start, but a war that he, without the US and Trump, will never be able to settle."
Speaking to reporters in Florida on February 18, Trump blamed Ukraine's authorities for the war and suggested they "could have made a deal" with Russia earlier.
Russia invaded Ukraine by land, air and sea on February 24, 2022, starting the largest military conflict in Europe since World War II.
Monday is day 1,272 of Russia's war on Ukraine.
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Source: CNN, TVP Info, IAR, PAP, Reuters