The U.S. will pause all military aid to Kyiv until President Trump determines that Ukrainian President Volodymyr Zelensky is making a good-faith effort toward peace negotiations with Russia, according to a White House official.

“The president has been clear that he is focused on peace. We need our partners to be committed to that goal as well. We are pausing and reviewing our aid to ensure that it is contributing to a solution,” a White House official said in a statement.

The White House didn’t announce under what conditions the U.S. would resume military aid, which drew from U.S. stockpiles, or whether it would be at the same pace. While there isn’t a clear understanding of what Washington wants from Kyiv, the move was the most demonstrative shift yet from Ukraine’s once top ally .

A senior administration official said all of Trump’s top national security advisers agreed with the decision to pause the aid after several meetings on the issue. Trump, enraged by Zelensky’s comments that he thought the end of the war was far away, felt the need to show he was serious about getting Ukraine to the peace table.

“The Ukrainians didn’t think we were serious,” the administration official said. “We had to make a demonstration.”

The official said Trump would decide what constitutes Ukraine showing that it is serious about peace talks, adding that it is unclear whether Zelensky signing a rare-minerals deal sought by the Trump administration would be enough to restart arms deliveries.

The U.S. decision comes days after a contentious meeting between Trump and Zelensky at the White House. The tense exchange Friday raised fears across Europe that the U.S. could be moving away from the wider Western alliance .

The U.S. has provided more than $120 billion in aid since Russia invaded Ukraine three years ago, including $67.3 billion in military aid. But it also provided intelligence and training and rallied the international community to support Ukraine. European nations contributed an additional $138 billion in military and humanitarian aid, according to the Kiel Institute for the World Economy, a research group in Germany.

The White House announcement underscored the growing divisions over foreign policy in Trump’s Republican Party, as some lawmakers cheered the move and others warned against it.

“Stopping support for Ukraine would jeopardize the stability of Europe and the free world,” Rep. Mike Lawler (R., N.Y.) said on X. “There are strong opinions on both sides of this issue, and I respect that. However, we must be pragmatic about the bigger picture and protect America’s interests abroad,” he added.

“Too bad Iran, North Korea and China are not pausing their military aid and economic support,” Rep. Don Bacon (R., Neb.) said about Russia’s supporters. “There is an invader and a victim, there is a democracy and a dictatorship, there is a country who wants to be part of the West and one who hates the West. We should be unambiguously for the good side,” he said.

Rep. Mary Miller (R., Ill.) backs Trump’s decision. “If Zelensky wants to continue fighting an endless war, let him do it himself,” she said on X. “The U.S. will no longer participate in this conflict that has led to the death of thousands. It’s time for peace!”

The decision also drew the ire of Ukraine’s supporters who warned pausing aid would prolong the war, not speed up the peace process.

“Stopping military aid to Ukraine is incredibly damaging to the United States and a sad day for American interests because it rewards our adversaries,” said Mykola Murskyj, director of advocacy at Razom for Ukraine. “I can hear the Champagne popping in Moscow, Beijing and Tehran.”

The Trump administration had previously stopped financing new weapons sales to Ukraine, another move that threatened Kyiv’s ability to fight at a critical time in its battle against Russian forces, current and former U.S. officials said.

Trump on Monday said Zelensky should be “more appreciative because this country has stuck with them through thick and thin.” A proposed mineral-rights deal between the U.S. and Ukraine would provide Kyiv with added security—even without explicit U.S. defense guarantees sought by Zelensky—because the U.S. would have a “presence there,” Trump said.

He lashed out at Zelensky for saying the war with Russia was likely to continue for some time. “This is the worst statement that could have been made by Zelenskyy, and America will not put up with it for much longer!” Trump wrote on his social-media platform Truth Social, using the different spelling of the Ukrainian leader’s name.

The U.S. weapons cutoff would leave Ukraine less able to withstand Russian attacks, analysts said.

“Ukraine wouldn’t surrender tomorrow or next week, but they would lose military capability gradually and at some point they would face defeat,” said Mark Cancian, a retired Marine Corps colonel and former U.S. official now at the Center for Strategic and International Studies, a Washington think tank.

Ukraine had been able to get weapons from the U.S. through several means, including Foreign Military Financing, which provides loans and grants for nations to buy weapons from U.S. defense companies, and the Ukraine Security Assistance Initiative, which allows the Pentagon to buy weapons for Kyiv but is currently out of funds. However, presidential drawdown authority, which the U.S. paused Monday, allowed the Defense Department to pull directly from its own stockpiles and had been the most significant tool for arming Ukraine.

The shutdown of financing new weapons sales began before Friday’s acrimonious meeting, during which Ukraine and the U.S. were supposed to sign a framework agreement for a mineral-rights deal but instead canceled the ceremony and a scheduled press conference. The Monday administration meetings, which led to the suspension of ongoing weapons shipments to Ukraine, emerged after the Friday blowup at the White House.

Without new U.S. military aid , Ukraine likely has enough weapons to keep fighting Russia at its current pace until the middle of this year, current and former Western officials say. That is the result in part of a surge of new weapons approved by the Biden administration in its final days.

Over the weekend, European leaders held a summit in London and agreed to form a coalition to forge a Ukraine peace plan to present to Trump that would include ground troops and military assets.

European allies and Ukraine’s own growing defense industry can make up for some of the potential shortfall, but an end to U.S. aid would cut Ukraine’s access to advanced weapons systems that are important to its strategy of countering Russia’s full-scale assault on its territory.

If the U.S. shutdown persists, Ukraine would lose its supply of some sophisticated weapons, including advanced air-defense systems, surface-to-surface ballistic missiles, navigation systems and long-range rocket artillery. The U.S. is the sole producer of some systems, including Army Tactical Missile Systems, or ATACMS, and M142 High Mobility Artillery Rocket System, or Himars , which give the Ukrainians the ability to strike far behind Russian lines.

Once those U.S. supplies run out, Ukraine’s ability to conduct longer-range strikes, and to protect its own rear positions, would suffer, officials and analysts say.

“Europe can step in to meet a fair amount of Ukraine’s need for artillery ammunition when combined with munitions already shipped by the U.S. early this year,” said Michael Kofman, a senior fellow at the Carnegie Endowment for International Peace who frequently visits Ukrainian front-line units. “The challenges will be more visible as we get into summer.”

Over time, Ukraine could find it harder to make longer-term plans for its military arsenal or to buy parts for its existing systems. Ukraine currently builds or finances about 55% of its military hardware. The U.S. supplies around 20%, while Europe supplies 25%.

Up until Monday’s announcement, the Trump administration had sent some of the weapons promised by the Biden administration, the Pentagon said, including “hundreds of Guided Multiple Launch Rocket Systems (GMLRS) and antitank weapons and thousands of artillery rounds that Ukraine is employing on the battlefield when they are available from our stocks.”

The last major new arms package for Ukraine was on Dec. 30, during the Biden administration, and included some advanced weapons like munitions for air-defense systems, Stinger missiles and other weapons. The package was worth $1.22 billion. No new weapons transfers have been announced since.

There are over $3 billion in funds to draw from existing stockpiles that have been authorized by Congress but not allocated by the administration, the congressional aide said.

Write to Nancy A. Youssef at nancy.youssef@wsj.com , Alexander Ward at alex.ward@wsj.com and Jared Malsin at jared.malsin@wsj.com