US President Joseph Biden suffered a defeat in Congress over the budget vote, but despite this, he promised continued aid to Ukraine.
At the last minute, additional military funding was cut from the budget in Congress.
The temporary measure, which was introduced to prevent a government shutdown, does not include $6 billion in military aid to Kiev, which has been a White House priority.
Hard-line Republicans oppose further military aid, and many openly oppose Biden's approach to the war.
Despite this, Biden said on Sunday that Ukraine can "count on" continued US support.
"Under no circumstances can we allow support for Ukraine to end," Biden said.
"I assure [Ukraine] that it will be so," he said regarding the continuation of war financing.
"I want to assure our American allies that you can count on our support, we will not leave," he added.
The US has already given at least $50 billion in military aid to Ukraine since Russia launched its invasion in February 2022.
Biden asked Congress for another $24 billion.
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In recent months, Washington has sent Kiev state-of-the-art weapons, including long-range missiles and Abrams tanks.
Additional military equipment was sent to Ukraine to continue the counter-offensive launched several months ago, which is slow and has not made any significant progress so far.
But Saturday's interim budget deal in Congress, which will fund the US federal government for 45 days, deprived Ukraine of further military funding for now.
Senior Senate leaders from both parties released a joint statement signaling their intention to "ensure that the U.S. government continues to provide" support to Ukraine in the coming weeks.
But that move, which followed only nine days after the Ukrainian President Volodymyr Zelensky was in Washington to ask for further support, reflects the growing opposition of some House Republicans to the war in recent months.
Republicans control the House of Representatives, and Democrats have a narrow majority in the Senate.
Both chambers must approve budget bills before they can be signed into law.
Florida Congressman Matt Goetz told reporters on Saturday that the approved budget was "somewhere between more than enough and too much," and Georgia Congresswoman Marjorie Taylor-Green said Kiev had already been given too much aid, saying that "Ukraine is not the 51st country " USA.
Their approach caused a stormy reaction from senators of the Democratic Party.
"I can't believe that people are going to turn their backs on Ukraine at this point," said Senator Mark Warner.
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Despite the disagreements, officials in Kiev are trying to present this new, 45-day US funding agreement as an "opportunity" for its diplomats to secure long-term support.
Ukraine's Foreign Ministry believes "the flow of US aid will not change" with three billion dollars in humanitarian and military support due to arrive, but admits it could affect "ongoing programs".
But one Ukrainian lawmaker, Alexei Goncharenko, admits the suspended funding is causing concern in Kiev.
"The vote in the US Congress is disturbing. The US said it would be with Ukraine as long as it takes, and now support for Ukraine has been cut from the budget.
"This is a wake-up call, not only for Ukraine, but also for Europe," he told the BBC.
The political turmoil in America is one of several symptoms of Western fatigue.
The growing skepticism of some republicans and the electoral victory of the populist, pro-Moscow party in Slovakia are worrying both Ukraine and the European Union.
In an interview with the BBC in Kyiv, Josep Borelj, European Commissioner for Foreign Affairs and Security Policy, said he was "concerned" by the latest decision by the US Congress on funding Ukraine.
"I don't know what will happen in the future.
"One thing is clear: for us Europeans, Russia's war against Ukraine is an existential threat, and we must react accordingly," he says.
In one of his daily addresses, Zelensky said that no one should "underestimate the resilience of Ukraine".
It is clear that Ukraine will continue to fight - with or without help from the West.
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