"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 a White House 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.
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 criticised him for being insufficiently grateful for Washington's backing in the war with Russia.
Oleksandr Merezhko, head of the Ukrainian parliament's foreign affairs committee, said Trump's military aid pause looked like the US president was pushing Kyiv to capitulate.
"On the surface, this looks really bad. It looks like he is pushing us towards capitulation, meaning (accepting) Russia's demands," he said.
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.
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 neighbour.
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.
The US Congress has approved $US175 billion ($A281 billion) in total assistance for Ukraine since Russia's invasion nearly three years ago, according to the nonpartisan Committee for a Responsible Federal Budget.
In December, right before leaving office, President Joe Biden announced an additional $US5.9 billion in security and budget assistance.
US assistance to Ukraine includes military aid, 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.
Some of the money sent by the US to Ukraine helps the country pay salaries of teachers, doctors and keeps the government running, allowing it to focus on fighting Russia's invasion.
In total the US has pledged $US31.7 billion worth of weapons aid to Ukraine. The vast majority - well over $US20 billion according to a Reuters analysis - has been shipped.
The announced pause in military aid applies mainly to aid that had been previously approved but not yet disbursed.
Trump has not approved any new aid under his own presidential authority since taking office and a new congressional aid package appears unlikely, at least in the near term.
Prior to the White House's decision to halt its aid, European nations were rallying around Zelenskiy and trying to hatch a peace plan.
"There are clearly a number of options on the table," British Prime Minister Keir Starmer's spokesman said.
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".
However, the Hungarian government, a vocal critic of EU sanctions against Moscow and the bloc's financial and military support for Ukraine, said of Trump's pause in military aid: "The US president and the Hungarian government share the same stance: instead of continuing weapons shipments and the war, a ceasefire and peace talks are needed as soon as possible."
with PA and AP
