Ukrainian President Volodymyr Zelensky on Wednesday directly appealed for long-term US support on his first foreign trip since the Russian assault, saying aid was not "charity" and had helped keep his country "alive and kicking".

Three hundred days after Russian leader Vladimir Putin attacked Ukraine, Zelensky enjoyed a hero's welcome on a lightning trip to Washington where President Joe Biden committed nearly US$1.8 billion in military supplies including, for the first time, the Patriot missile defence system.

Zelensky wore his trademark green military fatigues as he held talks at the White House and addressed Congress, where he handed over an Ukrainian flag fresh from the battlefield.

As Congress finalises a new package of US$45 billion, Zelensky voiced gratitude and said that the new year would mark the "turning point" in the fight against Russia.

"Against all odds and doom-and-gloom scenarios," Zelensky said in a speech that drew standing ovations, "Ukraine is alive and kicking".

"Your money is not charity. It's an investment in the global security and democracy that we handle in the most responsible way," Zelensky said.

House Speaker Nancy Pelosi, who welcomed Zelensky in one of her last acts before Republicans take control, likened the address to Winston Churchill's Christmas-time visit in 1941 days after the Pearl Harbor attack – and the Ukrainian leader was unafraid to encourage the parallels.

Comparing Ukrainian forces to Americans on the frontlines in World War II and the Revolutionary War, Zelensky said that Russians "burn and destroy everything" in a way "similar to the other tyranny" of Hitler.

"Ukraine holds its lines and will never surrender," Zelensky said to applause.

Ukraine enjoys wide support in the United States but several hard-right Republicans have been critical, considering the aid a giveaway of taxpayer money.

Kevin McCarthy, likely to become speaker next month after a narrow Republican election victory, has warned he will not give a "blank cheque" to Ukraine, a comment seen as attempting to mollify his right flank. (AFP)