Senate Republicans have blocked a move to pass an aid bill for Ukraine after failing to secure border compromises they sought in exchange.
The $110bn (£87.3bn) package included $61bn for Ukraine, as well as funds for Israel and aid for Gaza.
The White House has warned that US funds for Ukraine could soon run out.
A Ukrainian official said that that failure to secure more US aid would mean a "very high possibility" that the war will be lost to Russia.
While Republican members are generally in favour of aid to Ukraine, some have sought to use the issue as a way address mounting domestic concerns over the US southern border.
Senators - including every single Republican - voted 51 to 49 against advancing the bill, with 60 votes needed. The vote throws uncertainty into the future of aid for Ukraine and sends lawmakers back to the negotiating table with just days to go until Congress has scheduled winter break.
Republicans are insisting that any aid to Ukraine be tied to sweeping US immigration and asylum reforms.
Earlier on Wednesday, US President Joe Biden said he was "willing to make significant compromises on the border" in order to get the aid bill passed.
"This cannot wait," he said, adding that "Republicans in Congress are willing to give Putin the greatest gift he could hope for".
Also on Wednesday, the Biden administration announced $175m in new security assistance for Ukraine from the supply of funding that has already been approved. The package includes ammunition, including missiles and artillery shells, as well as "equipment to protect critical national infrastructure", the US Department of Defense said in a news release.
Concerns over the future of the $110bn package grew on Tuesday after a classified briefing for lawmakers aimed at shoring up support for new funds broke down spectacularly.
Senators shouted at each other over border security and at least a dozen Republicans walked out.
Ukrainian President Volodymyr Zelensky also cancelled a virtual briefing with lawmakers over a "last-minute matter", Senate Majority Leader Chuck Schumer said on Tuesday, without providing further detail.
The package already includes provisions for border security, but Republican demands for additional changes to asylum rules has complicated negotiations with Democrats.
Ahead of the failed vote to bring the package to the floor, Mr Schumer delivered an emotional plea to his colleagues on the Senate floor, telling them that the vote was an important "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 said.
On Tuesday, the head of Ukraine's presidential office, Andriy Yermak, told an audience at the US Institute of Peace that a failure to secure more US aid would mean a "very high possibility" that the war will be lost and that it will be "impossible to continue to liberate" Russian-held areas.
In Ukraine, dimming prospects for additional aid have led to a darkening mood among some parts of the population.
"Of course we need support, we are protecting the whole of Europe," Tetyana, a Kyiv resident whose son is on the frontline, told the OceanNewsUK this week. "We need more weapons because our children are dying."
The Senate bill needed nine Republican votes to advance - a threshold that was ultimately too high. Some Democrats expressed frustration at their Republican colleagues.
"The Ukrainians are on the frontlines fighting for democracy," Massachusetts Democrat Elizabeth Warren told reporters. "This is about freedom".
Several Senators said that while more negotiations would follow, it is unclear whether any progress can be achieved before Congress breaks for the holidays next week.
Mr Schumer, for his part, said "we'll see" when asked whether he believed Senators could come to an agreement before the break, even though he believed Mr Biden had presented a "good strong plan".
One of the Republicans who opposed the package, South Carolina's Lindsey Graham, said he did not believe any solution would be possible in that timeframe.
Mr Graham added that he believed Mr Biden would ultimately need to negotiate more, saying that "it's going to take his leadership or we are stuck."
"They know what we want," he said. "I'm hopeful we can get the border part in a place where we can go for a bill."
Even if it had passed in the Senate, the package still would have faced an uphill battle in the House of Representatives.
House Speaker Mike Johnson said on Tuesday that he has told the Senate he cannot pass any Ukraine aid without the inclusion of significant border security measures.
Since Russia invaded Ukraine in February 2022, Congress has approved over $110bn in military and economic aid to Ukraine, most of which has already been distributed.
In a letter to Mr Johnson released publicly earlier this week, White House budget director Shalanda Young said that the US would be unable to get more weapons and equipment to Ukraine "by the end of the year" without Congressional action.
Ukrainian officials have repeatedly stressed that they see US aid as vital to the ability of the country's forces to resist the Russians and re-take occupied territory.
Another senator to vote against the aid bill was independent Senator Bernie Sanders, who had earlier in the day expressed reservations that the legislation included billions in military aid for Israel.
"I do not believe we should be appropriating over $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 the Gaza strip, which has so far killed thousands of civilians.
Mr Sanders, who is a long-time critic of Mr Netanyahu, added: "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."
