President Joe Biden has vowed continued US support for Ukraine, after further military funding was excluded from a last-minute congressional budget deal.
The temporary measure, pushed through to avert a government shutdown, did not include $6bn (£4.92bn) in military aid for Kyiv - a top White House priority.
Hardline Republicans oppose further military aid, with many openly opposing Mr Biden's approach to the war.
But on Sunday Mr Biden said Ukraine could "count on" US support.
"We cannot, under any circumstances, allow US support to Ukraine to be interrupted," Mr Biden said.
"I can reassure [Ukraine] we'll get there, that we're going to get it done," he said on restoring funding for the war. "I want to assure our American allies... that you can count on our support, we will not walk away."
The US has already supplied some $46bn (£37bn) in military aid to Ukraine since Russian launched its full scale invasion in February 2022.
President Biden has requested another $24bn (£19bn).
And in recent months the US has sent state of the art equipment to Kyiv - including long-range missiles and Abrams tanks. It comes as Kyiv's forces continue to launch a slow moving counter-offensive in the south of the country.
But Saturday's temporary budget agreement - which will fund the US federal government for 45 days - stripped out continued military funding for the time being.
Senior Senate leaders from both parties released a joint statement signalling their intention to "ensure the US government continues to provide" support to Ukraine in the coming weeks.
But the move - which came just nine days after President Volodymyr Zelensky flew to Washington to plead for further support - reflects increasing opposition from hard-right Republicans in the House of Representatives to the war in recent months.
Republicans control the House of Representatives, with Democrats enjoying a wafer-thin majority in the Senate. Both need to approve legislation on the budget before it is signed into law.
Florida congressman Matt Gaetz told reporters on Saturday that funding "already authorised out of this Congress is somewhere between more than enough and way too much".
And Georgia Representative Marjorie Taylor-Green said far too much aid had already been allocated to Kyiv, saying "Ukraine is not the 51st state".
Their approach provoked a furious reaction from Democratic Party senators.
"I can't believe people are going to walk away from Ukraine at this moment in time," Senator Mark Warner said.
Despite the row, officials in Kyiv have sought to frame this new 45-day funding agreement in the US as an "opportunity" for its diplomats to secure longer-term support. It's more like an unwanted deadline.
Ukraine's foreign ministry says the "flow of US aid won't change" with $3bn of humanitarian and military support set to still arrive, but it concedes "ongoing programmes" might be affected.
But one Ukrainian MP, Oleksi Goncharenko, admitted that the suspended funding was causing concern in Kyiv.
"The vote in US Congress is disturbing. The US said they would be with Ukraine as long as it takes and now see how support of Ukraine is excluded from the stop-gap deal. This is the sign of alarm, not only for Ukraine, but for Europe, too," he told the BBC.
This political turmoil is one of several symptoms of Western fatigue. The growing scepticism from some Republicans and the recent election victory for a populist, pro-Moscow party in Slovakia are concerning for both Ukraine and the European Union.
In an interview with the BBC in Kyiv, the EU's most senior diplomat, Josep Borrell, said he was "worried" by the latest decision on funding for Ukraine by the US Congress.
"I don't know what's going to happen in the future," he told the BBC. "One thing is clear: to us Europeans the war of Russia against Ukraine is an existential threat, and we must react accordingly."
In his daily address from Kyiv, President Zelensky said no-one should be able to "turn off Ukraine's resilience".
It is clear the country will continue to fight with or without help from the West.
Ukraine knows what it would overwhelmingly prefer.