President Volodymyr Zelenskiy has told the US Congress that aid to Ukraine is an investment in democracy, not charity, and invoked American victory over the Nazis in a key World War II battle to press for continued assistance.
Zelenskiy, on his first foreign wartime visit, wore his trademark olive green pants and jumper. He earlier met US President Joe Biden, who urged support to keep flowing in 2023, when congressional approval for Ukrainian aid will be harder.
The United States has sent about $US50 billion ($A75 billion) in assistance to Kyiv as Europe's biggest land conflict since 1945 drags on, killing tens of thousands, driving millions from their homes and reducing cities to ruins. On Wednesday, it pledged to provide Patriot missiles.
But some Republicans, who will take control of the House of Representatives from Democrats on January 3, have expressed concerns about the price tag. They could hold up billions of dollars in war aid starting next month.
"Your money is not charity. It is an investment in the global security and democracy that we handle in the most responsible way," Zelenskiy told a joint session of the US Senate and House of Representatives, speaking in English.
The world was too interconnected to allow any country to stand aside and feel safe, he added.
Zelenskiy's speech was aimed at softening the concerns of Republicans when they take over Congress and keeping US public opinion on his side with reminders of the hardships his people face.
Members of Congress stood, cheered, applauded and shook Zelenskiy's hand as he entered the chamber, with several wearing the colours of the Ukrainian flag, blue and yellow.
He received multiple standing ovations from an audience that also included members of the Ukrainian diaspora and religious figures.
Referencing former US President Franklin D Roosevelt, who served between 1933 and 1945, and efforts to liberate Europe from Nazi occupation, Zelenskiy also appealed to Americans as they gathered with family for Christmas.
"Just like the brave American soldiers, which held their lines and fought back Hitler's forces during the Christmas of 1944, brave Ukrainian soldiers are doing the same to Putin's forces this Christmas," he said.
The Battle of the Bulge, which began in December 1944, was Hitler's final major attempt to push back Allied forces. Poor weather hampered initial US efforts to halt the offensive, leading to fatalities and threatening to divide the allies, who ultimately prevailed.
Earlier, Zelenskiy was greeted by Biden and first lady Jill Biden on the White House lawn and the two leaders appeared together at the Oval Office, followed by a news conference.
"As we head into the New Year, it's important for the American people, and for the world, to hear directly from you Mr President about Ukraine's fight and the need to continue to stand together through 2023," Biden said.
Zelenskiy said: "The United States will stand up for our shared values, the values of freedom."
US Secretary of State Antony Blinken also announced the United States would provide another $US1.85 billion ($A2.76 billion) in military aid for Ukraine including the highly advanced Patriot air defence system to help it ward off barrages of Russian missiles.
The Patriot missile is deemed to be one of the most advanced US air defence systems, offering protection against attacking aircraft as well as cruise and ballistic missiles.
Zelenskiy has repeatedly called on the West to supply more advanced weaponry, ranging from modern battle tanks to missile defence systems, but Western allies have been cautious, keen to minimise any risk of provoking wider conflict with Russia.
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