Ukrainian President Volodymyr Zelensky told the U.S. Congress that aid to his country was an investment in democracy and "not charity" as he invoked battles against the Nazis in World War Two to press for more assistance in the war against Russia.

Zelensky's comments on Wednesday come as Republicans - some of whom have voiced increasing scepticism about sending so much aid to Ukraine - are set to take control of the U.S. House of Representatives from Democrats on Jan. 3.

"Your money is not charity. It is an investment in the global security and democracy that we handle in the most responsible way," Zelensky told a joint meeting of the U.S. Senate and House of Representatives, speaking in English.

Zelensky sought to win sceptics over with a speech that tied the future of the war, and freedom, to America’s commitment to Ukraine.

He said his country had defied expectations that it would not be able to hold out for more than a few weeks against Russia.

“Against all odds and doom and gloom, Ukraine didn’t fall. Ukraine is alive and kicking,” he said.

That survival had produced different kinds of victories, he said.

“We defeated Russia, in the battle for minds of the world. We have no fear. Nor should anyone in the world have it. Ukraine’s gained this victory and it gives us courage, which inspires the entire world.”

But, Zelensky said, the struggle on the battlefield remained, and America was central to what happened there.

“I know that everything depends on us, on the Ukrainian armed forces. Yet so much depends on the world. So much in the world depends on you,” he told Congress.

“Russia could stop its aggression, really, if it wanted to. But you can speed up our victory.”