“It’s very important that you are with us and that we speak to America together,” Zelensky said in Brussels, standing alongside von der Leyen. “It’s crucial that Europe is as united now as it was at the very beginning—as it was in 2022,” when Russia staged its large-scale invasion of Ukraine, Zelensky said. “This unity really helps encourage real peace and it must stay strong.”

Zelensky also thanked Trump for saying in recent conversations with European leaders that was open to providing security guarantees to Ukraine.

“For everyone in Europe this is a significant change, but there are no details how it would work and what America’s role will be, what Europe’s role will be, what the EU can do,” Zelensky said. “This is our main task: We need security to work in practice.”

European officials said they expect Zelensky to face pressure from Trump to accept a deal outlined by Putin on Friday at their meeting in Alaska. Under that proposal, Ukraine would withdraw from and surrender its eastern Donbas region, including parts of the Donetsk region it still controls, according to officials familiar with the discussions. In exchange, Russia would freeze the conflict along the current contact line in Zaporizhzhia and Kherson, the officials said.

The officials said the main task for Zelensky on Monday will be to persuade Trump that there are other, better alternatives for meeting the U.S. president’s top goal of ending the war quickly.

Were Zelensky simply to reject Putin’s demands, the resulting impasse could lead to a fresh tensions between Trump and Zelensky. When Zelensky visited the White House in February he got into a shouting match with Trump and Vice President JD Vance.

Zelensky and European leaders have long insisted that Ukraine’s territorial integrity and sovereign choices should be respected in peace negotiations. European officials privately acknowledge that Ukraine won’t regain all of its territory, but they say Russian occupation of parts of Ukraine should be as small as possible and should never be recognized legally as Russian territory.

Zelensky noted Sunday that Ukraine’s constitution forbids surrendering territory or trading land. He would likely face huge pushback domestically if he ceded territory that Russia’s military hasn’t been able to conquer.

“Putin has many demands, but we do not know all of them,” Zelensky said. “It will take time to go through them all. It’s impossible to do this under the pressure of weapons,” so a cease-fire is necessary now, followed by quick work on a final deal, he said.

Write to Daniel Michaels at Dan.Michaels@wsj.com and Laurence Norman at laurence.norman@wsj.com