Ukrainian President Volodymyr Zelenskiy has faced a sceptical reception from key Republicans during a trip to Washington to seek more military support against Russia, but he won a pledge at the White House that the US has his back.
Republicans have been reluctant to sign off on a funding request from Democratic President Joe Biden under which Ukraine would receive $US61.4 billion ($A93.6 billion).
House of Representatives Speaker Mike Johnson, a Republican, said after meeting Zelenskiy that Biden's administration must provide more detail about how the money would be used.
"What the Biden administration seems to be asking for is billions of additional dollars with no appropriate oversight, no clear strategy to win and with none of the answers that I think the American people are owed," he said.
Biden told Zelenskiy he would not walk away from Ukraine and neither would the American people, but he warned lawmakers they risked handing a victory to Russian President Vladimir Putin.
"Putin is banking on the United States failing to deliver for Ukraine," he said during a news conference with the Ukrainian leader.
"We must ... prove him wrong."
Earlier in the Oval Office, Biden told Zelenskiy "we're gonna stay at your side", and said Congress needed to pass aid "before they give Putin the greatest Christmas gift they could possibly give him".
Zelenskiy said Ukraine was making progress in becoming more self-sufficient and less dependent on aid, and he stressed his country's success against Russia had an impact on other European nations as well.
"Thanks to Ukraine's success, success in defence, other European nations are safe from the Russian aggression," he said.
Heading into winter, with tens of thousands of Ukrainians dead, a yawning budget deficit and Russian advances in the east, Zelenskiy is asking Washington to provide badly needed support.
Zelenskiy was met with sustained applause as he entered a closed-door meeting with US senators, and the chamber's Democratic and Republican leaders pledged their support.
Some Republicans, particularly those with the closest ties to former president Donald Trump, oppose more Ukraine aid and are asking about the war aims and how US money is being spent.
They say any further money must be paired with changes to immigration policy - an exceptionally divisive issue in US politics.
Other Republicans questioned whether additional aid would help Ukraine defeat Russia after a summer offensive that has failed to yield clear gains.
"I know everyone wants Ukraine to win, I just don't see it in the cards," Republican senator Ron Johnson said.
Democrats in Congress accused their political opposition of aiding Putin.
"The one person happiest right now about the gridlock in Congress is Vladimir Putin. He is delighting in the fact that Donald Trump's border policies are sabotaging military aid to Ukraine," Democratic Senate leader Chuck Schumer said.
Speaker Johnson said he would not act until the Senate passed legislation.
"I implore them to their job because the time is urgent and we do want to do the right thing," he told reporters.
Newly declassified US intelligence showed "Russia seems to believe that a military deadlock through the winter will drain Western support for Ukraine", said Adrienne Watson, spokeswoman for the White House National Security Council.
Ukraine was having success stopping Russian forces, but Putin continued to order his troops forward despite heavy losses of troops and equipment since October, she said.
The White House told Congress on December 4 the government would no longer have funding to provide more weapons for Ukraine after the end of the year.
Congress has approved more than $US110b for Ukraine since Russia's February 2022 invasion but no new funds since Republicans took over the House from Democrats in January.
The United States could not turn the tide of war in Ukraine by pumping tens of billions more dollars into the country, the Kremlin said on Tuesday.
The war has cost Russia 315,000 dead and injured troops, nearly 90 per cent of the personnel it had before the conflict began, according to a source familiar with a declassified US intelligence report.
There are just three days before Congress recesses for the year on Friday, and Republicans in the House have until refused to pass a spending package bill that contains the $US61.4b in Ukraine aid without fiercely disputed changes to US immigration.