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. (BBC)