Military aid from Kyiv's most significant ally could plunge in the coming weeks.
“We are running out of money and soon running out of time” to assist Ukraine, the White House has said, amid rancour over the budget agreement.
“If Congress does not act, by the end of the year we will run out of resources to deliver more weapons and equipment to Ukraine and to supply material from US military stockpiles,” wrote White House Budget Director Shalanda Young in a letter addressed to Mike Johnson, head of the House of Representatives, where there is a Republican majority.
This letter kicked the ball back to the right-wingers, where calls to detach from the Ukraine war and stop spending US taxpayers' money are growing.
During a press conference, US National Security Adviser Jake Sullivan said "Congress must decide whether to continue supporting the fight for freedom in Ukraine... [or] ignore the lessons we have learned from history and allow [Russian President Vladimir] Putin to prevail.”
Young's letter comes at a difficult time for Kyiv, with its summer counteroffensive failing to achieve any significant results. Russian forces, meanwhile, have launched repeated assaults that are pilling on the pressure, especially around Avdivka in the east.
“There is no magic funding available to deal with the emergency. We are running out of money and soon to run out of time,” wrote the Budget Director.
Democratic President Joe Biden asked Congress on 20 October to vote for exceptional funding of more than $100 billion (€92.5bn) to respond to international emergencies, such as assisting Israel and Ukraine, standing up to China and responding to the arrival of migrants at the southern US border.
More than $60bn (€55.5bn) of this sum was intended for Kyiv as it battles the Russian invasion launched in February 2022.
The time is now
Ukrainian President Volodymyr Zelenskyy will address US senators on Tuesday during a classified and closed video conference, according to the leader of the Democratic majority in the Senate, Chuck Schumer.
He urged all senators to attend the briefing "so we can hear directly from him what precisely is at stake."
The White House wants to maintain funding for Ukraine at least until the presidential election in November 2024, which could pit Biden against former President Donald Trump.
“Putin will not commit to peace before seeing the result of our election,” a senior US diplomatic official was quoted as recently saying by AFP, a French news agency.
Washington has been in budgetary limbo for months, due to endless political turbulence.
The Congress of the world's leading power - made up of the Senate with a Democratic majority and the House of Representatives with a Republican majority - has still not voted on a budget for the fiscal year which began on October 1.
Government is only functioning thanks to an emergency extension which will expire in mid-January.
When Biden solemnly requested his enormous budget package, the House of Representatives found itself in chaos due to dissension within the Republican Party.
It has since acquired a leader Speaker Mike Johnson, which allowed the resumption of budgetary debates, which promise to be no less difficult.
The head of conservative lawmakers is demanding a tightening of migration policy amid surging migrant arrivals at the Mexican border in exchange for support of a new package for Ukraine.