The White House issued a stark warning on Monday that US aid for Ukraine will run out by the end of the year, saying that if Congress fails to agree funding, it will "kneecap" Kyiv's fight against Russia.

"We are out of money -- and nearly out of time," White House budget director Shalanda Young said in a blunt letter to Republican House Speaker Mike Johnson, reflecting President Joe Biden's growing frustration with a congressional stalemate.

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

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.

"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," Young wrote.

"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."

US aid packages to Ukraine had already become smaller due to the funding squeeze, added Young, the head of the White House's Office of Management and Budget.

But 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.

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

But Congress has been paralyzed for months by Republican infighting, with hard-right lawmakers particularly opposing any further assistance for Kyiv as the war drags into its third year.

Congress 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. (AFP)