Washington DC - The White House warned Monday that US aid for Ukraine will run out by the end of the year, and Russian President Vladimir Putin could win the war if Congress fails to agree to fresh funding.

President Joe Biden's budget director, Shalanda Young, said in a blunt letter to Republican House Speaker Mike Johnson that if military assistance dries up, it would "kneecap" Kyiv's fight against the Russian invasion.

Democrat Biden asked Congress in October for a huge $106 billion national security package, including military assistance for Ukraine and for Israel's war against Hamas, but the funding has been mired in divisions on Capitol Hill.

"There is no magical pot of funding available to meet this moment. We are out of money — and nearly out of time," wrote Young.

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"I want to be clear: without congressional action, by the end of the year, we will run out of resources to procure more weapons and equipment for Ukraine," she added.

"Cutting off the flow of US weapons and equipment will kneecap Ukraine on the battlefield, not only putting at risk the gains Ukraine has made but increasing the likelihood of Russian military victories."

National Security Advisory Jake Sullivan went further, suggesting that voting against aid for Ukraine was effectively voting to make it easier for Russia to succeed.

"Congress has to decide whether to continue to support the fight for freedom in Ukraine...or whether Congress will ignore the lessons we've learned from history and let Putin prevail," Sullivan told reporters at the White House.

"It is that simple. It is that stark a choice."

Ukraine has been desperately pushing for more Western aid as Russian forces step up attacks in the winter after Kyiv's counteroffensive failed over the summer.

But speaker Johnson, who took office in October after his predecessor was ousted in a right-wing coup, gave the letter a cool response.

"The Biden administration has failed to substantively address any of my conference's legitimate concerns about the lack of a clear strategy in Ukraine," Johnson said on X, formerly Twitter, on Monday.

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Johnson also repeated the Republicans' insistence on tying any Ukraine aid to changes in US policy on the southern border with Mexico as the number of migrant arrivals surges.

Casting Putin and Hamas as twin forces trying to "annihilate" neighboring democracies, Biden has sought to tie $61 billion for Ukraine with $14 billion for Israel in the aid package he demanded in October, along with funding for the border.

Congress only narrowly averted a chaotic government shutdown over the Thanksgiving holiday, but the deal to keep the lights on until mid-January left out aid to Ukraine and Israel.

Ukraine's frontline has largely remained static for the last year despite a massive push by Ukrainian forces this summer with Western military hardware.

The United States has already allocated $111 billion for Ukraine since Russia invaded in February 2022, including $67 billion for military procurement, Young said.

European countries are also facing challenges in securing funding for Ukraine as fatigue with the war sets in.

Fears are growing in Washington that Putin may be content to sit out the situation until next year's presidential election, a probable replay of 2020's contest between Biden and Donald Trump.

Polls show a growing number of voters saying the United States is doing too much to help Kyiv.

"I think my expectation is that Putin won't make peace or a meaningful peace before he sees the result of our election," a senior State Department official told reporters last week.

The official would not say why, but Moscow is widely seen as favoring a return by Trump, who has praised the Russian leader and questioned US aid for Ukraine.

Trump was impeached by the House in 2019 for trying to pressure Ukrainian President Volodymyr Zelensky into digging up what he alleged was compromising information on Biden's son Hunter, who had once worked for a Ukrainian energy company.

Trump was acquitted by the Senate in that case, the first of his two impeachments.