The move comes after Trump upended US policy on Ukraine and Russia upon taking office in January, adopting a more conciliatory stance towards Moscow - and after an explosive confrontation with Zelenskiy at the White House on Friday in which Trump criticized him for being insufficiently grateful for Washington's backing in the war with Russia.

"President Trump 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," said the official on Monday, speaking on the condition of anonymity.

The White House had no immediate comment on the scope and amount of aid affected or how long the pause would last. The Pentagon could not provide further details.

Zelenskiy's office did not immediately respond to a Reuters request for comment nor did the Ukrainian Embassy in Washington.

On Monday, Trump again said Zelenskiy should be more appreciative of American support after earlier responding angrily to an Associated Press report quoting Zelenskiy as saying the end of the war is "very, very far away."

"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 Truth Social, using an alternative spelling of the Ukrainian leader's name.

Minerals deal not dead, says Trump

Since Russia's invasion three years ago, the US Congress has approved $175 billion in total assistance for Ukraine, according to the nonpartisan Committee for a Responsible Federal Budget.

The Trump administration inherited $3.85 billion worth of Congressionally-approved authority to dip into US arms stocks for Ukraine, but given the growing row between Washington and Kyiv, it was already unlikely for that assistance to be used.

Monday's move goes beyond the no-new aid stance Trump had taken since assuming office - and appears to halt deliveries of military equipment approved by Biden including munitions, missiles and other systems.

But Trump also suggested on Monday that a deal to open up Ukraine's minerals to US investment could still be agreed despite his frustration with Kyiv, as European leaders floated proposals for a truce in Russia's war with its neighbor.

The Trump administration views a minerals deal as America's way of earning back some of the tens of billions of dollars it has given to Ukraine in financial and military aid since Russia invaded three years ago.

When asked on Monday if the deal was dead, Trump said at the White House: "No, I don't think so."

Trump described it as a "great deal for us" and said he would give an update on the situation on Tuesday night when he addresses a joint session of Congress.

Security guarantees

In an interview on Fox News, Vice President JD Vance called on Zelenskiy to accept the minerals deal.

"If you want real security guarantees, if you want to actually ensure that Vladimir Putin does not invade Ukraine again, the very best security guarantee is to give Americans economic upside in the future of Ukraine," Vance said.

Zelenskiy has made clear that a ceasefire must carry explicit security guarantees from the West to ensure that Russia, which holds about 20 percent of its land, does not attack again. Trump has refused to give any such guarantees.

Beyond the military portion, US assistance to Ukraine also includes budgetary assistance, largely delivered through a World Bank trust fund, and other funds that had been delivered through the US Agency for International Development, which has been throttled by the Trump White House.

The US decision left many questions unanswered, including whether munitions for the already delivered weapons systems can now be supplied or if the US would still share intelligence with Ukraine on target identification and missile launches.

Key members of congressional oversight committees were not notified of the decision, including those on the Senate Armed Services Committee, a person with knowledge told Reuters.

Razom for Ukraine, a Ukrainian advocacy group, condemned the White House's decision on aid. "By abruptly halting military assistance to Ukraine, President Trump is hanging Ukrainians out to dry and giving Russia the green light to keep marching west," the group said in a statement.

Europe seeks peace plan 

Prior to the White House's decision to halt its aid, European nations were rallying around Zelenskiy and trying to hatch a peace plan.

Privately, and sometimes publicly, officials are fuming at what they see as a betrayal of Ukraine, which had enjoyed staunch support from Washington since Russia's invasion.

France, Britain and potentially other European countries have offered to send troops to Ukraine in the event of a ceasefire - something Moscow has already rejected - but say they would want support from the US, or a "backstop."

"There are clearly a number of options on the table," British Prime Minister Keir Starmer's spokesman said.