Ukrainian President Volodymyr Zelensky made an impassioned plea for continued US military support Monday in a Washington speech, warning that failure to help his country defeat Russian invasion is fulfilling the Kremlin's "dreams" of wrecking democracy in Europe.

Addressing an audience of US officers at the National Defense University, Zelensky said Ukraine is fighting not just for its own existence but in defense of the freedoms that opened up across Europe in the wake of the Soviet collapse.

In a rebuke to Republicans in Congress who have turned against US funding for the Ukrainian war effort, Zelensky said politicians should not "betray the soldier" -- and he said the drying up of US aid was being cheered by Russian President Vladimir Putin.

"If there's one inspired by unresolved issues on Capitol Hill its just Putin and his sick clique," he said. "They see their dreams come true when they see delays."

"You can count on Ukraine and we hope just as much to be able to count on you," Zelensky said. "Putin must lose."

The Ukrainian leader, who wore his trademark army style green sweatshirt, emblazoned with the words "I'm Ukrainian," flew into Washington after a round of diplomacy this weekend in Argentina.

He was due to meet Tuesday with President Joe Biden and congressional leaders from both parties, including Republican House of Representatives Speaker Mike Johnson.

White House National Security Council spokesman John Kirby said the visit came at "a critical time" and that Biden would make it clear he was "standing firm" on his bid to get Ukraine the aid it needs to resupply its troops and expand efforts to drive back Russian forces.

Zelensky was also meeting the heads of the International Monetary Fund and World Bank as he seeks to shore up his embattled nation's economy in the midst of the all-out war. The IMF announced the release of a new $900 million tranche in an ongoing longterm loan.

- Republicans sour on Ukraine -

Throughout the bloody conflict, which has seen swaths of Ukraine destroyed and millions driven from their homes, Ukrainian forces have depended heavily on a US-led coalition of countries delivering tens of billions of dollars in ammunition, weaponry, and economic and social aid.

Now the flow of US aid -- described by Biden as part of an existential fight between the democratic world and Putin's aggressive autocracy -- is on the verge of drying up.

Republican senators last week blocked a White House request for $106 billion in emergency aid primarily for Ukraine and Israel.

Conservatives said they would refuse the package for the close foreign allies if Democrats and the White House did not also agree to far-ranging immigration reforms targeting security on the politically sensitive US-Mexican border.

More broadly, the Republican right-wing, led by 2024 presidential candidate Donald Trump, has dramatically soured against Ukraine's cause.

"What's in America's best interest is to accept Ukraine is going to have to cede some territory to the Russians and we need to bring the war to a close," Senator JD Vance, a close Trump ally, said Sunday.

He dismissed as "preposterous" White House warnings that allowing Russia to win in Ukraine would put other eastern European countries, including NATO members, at risk.

There should be no "blank check" for Ukraine, Vance said. "You need to articulate what the ambition is. What is $61 billion going to accomplish that $100 billion hasn't?"