Senate Republicans have blocked a move to adopt an assistance bill for Ukraine after failing to achieve the border compromises they needed in exchange.
The $110 billion (£87.3 billion) plan contained $61 billion for Ukraine, as well as funding for Israel and help for Gaza.
Republicans are asking that any help to Ukraine be linked to major immigration and refugee reforms in the United States.
The White House has warned that US aid to Ukraine may soon run out.
Senators voted 51 to 49 against moving the bill, with 60 votes required. The vote casts doubt on the future of Ukraine aid and brings legislators back to the negotiation table with only days until Congress's scheduled Christmas vacation.
Every Republican Senator voted no, as did independent Senator Bernie Sanders, who had expressed qualms earlier in the day over the legislation's inclusion of billions in military aid to Israel.
"I do not believe we should be appropriating more than $10 billion for the right-wing extremist Netanyahu government to continue its current military approach," Mr Sanders said, referring to Israeli Prime Minister Benjamin Netanyahu's ongoing campaign in Gaza, which has killed thousands of civilians.
Mr Sanders, a long-time critic of Mr Netanyahu, adding, "What the Netanyahu government is doing is immoral, it is in violation of international law, and the United States should not be complicit in those actions."
Earlier on Wednesday, US Vice President Joe Biden stated that he was "willing to make significant compromises on the border" in order to get the aid bill passed.
"This cannot wait," he stated. He went on to say that "Republicans in Congress are willing to give Putin the greatest gift he could hope for" .
Also on Wednesday, the Biden administration pledged $175 million in fresh security support for Ukraine from previously approved funding. The shipment contains ammunition, including missiles and artillery rounds, as well as "equipment to protect critical national infrastructure," according to a news release from the US Department of Defence.
Concerns over the $110 billion package's survival heightened on Tuesday after a private briefing for senators intended at bolstering support for fresh funding collapsed miserably.
Senators yelled at each other on border security, and at least a dozen Republicans walked out.
Ukrainian President Volodymyr Zelensky also cancelled a virtual briefing with senators due to a "last-minute matter," Senate Majority Leader Chuck Schumer said on Tuesday, without providing any details.
The bill already includes border security elements, but Republican demands for additional changes to asylum laws have strained negotiations with Democrats. While the party's members are strongly in favour of aid to Ukraine, some have attempted to utilise the issue to address growing home worries about the US's southern border.
Before the unsuccessful vote to send the package to the Senate floor, Mr. Schumer made an emotional pitch to his colleagues, telling them that the vote was a "moment in history" and that they should "rush to the defence of democracy" in Ukraine.
"You can be sure that Vladimir Putin is watching closely," he told reporters.
The Senate bill required nine Republican votes to advance, which proved too high in the end. Some Democrats were irritated with their Republican counterparts.
"The Ukrainians are on the frontlines fighting for democracy," Massachusetts Democrat Elizabeth Warren told reporters. "This is about freedom" .
Several Senators stated that while more conversations would take place, it is uncertain whether any progress can be made before Congress adjourns for the holidays next week.
Mr. Schumer, for his part, said "we'll see" when asked if Senators could reach an agreement before the break, while believing Mr. Biden had offered a "good strong plan."
Lindsey Graham of South Carolina, one of the Republicans who opposed the deal, said he did not believe any compromise could be reached in that period.
Mr Graham went on to indicate that he believed Mr Biden would eventually need to negotiate more, stating that "it's going to take his leadership or we're stuck."
"They know what we want," he remarked. "I'm hopeful we can get the border part in a place where we can go for a bill."
Even if it had cleared the Senate, the package would have faced an uphill battle in the House of Representatives.
House Speaker Mike Johnson said on Tuesday that he has reminded the Senate that any Ukraine help must include major border security measures.
Since Russia's invasion of Ukraine in February 2022, Congress has approved over $110 billion in military and economic aid to Ukraine, the vast majority of which has already been disbursed.
In a letter to Mr. Johnson that was made public earlier this week, White House budget director Shalanda Young stated that the US would be unable to transfer more weapons and equipment to Ukraine "by the end of the year" unless Congress took action.
Ukrainian leaders have frequently stated that US assistance is critical to the country's forces' capacity to repel Russian forces and retake occupied territory.
Andriy Yermak, the head of Ukraine's presidential office, warned an audience at the US Institute of Peace on Tuesday that failing to gain more US aid would mean a "very high possibility" that the war would be lost and it would be "impossible" to continue liberating Russian-held territory.
Dimming hopes for additional aid in Ukraine have contributed to a downbeat atmosphere among some segments of the public.
"Of course, we need help; we're protecting the whole of Europe," Tetyana, a Kyiv resident whose son is on the frontlines, told the BBC this week. "We need more weapons because our children are dying." /BGNES
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