US President Joe Biden invited his Ukrainian counterpart Volodymyr Zelenskyy to Washington because he determined the war in Ukraine had entered a new stage.

John Kirby, the strategic communications coordinator at the National Security Council, said this on CNN.

"The President really believes that as we approach winter, as we enter... a new phase in this war, Mr. Putin's aggression, that this is a good time for the two leaders to sit down face to face and talk," Kirby said.

The talks are expected to center on continued US military and economic assistance, along with sustaining sanctions on Russia and supporting the Ukrainian people.

But Kirby said they would also discuss how the war might end, a sensitive topic that has been the subject of some disagreement among Western leaders.

"I have no doubt they are going to talk about President Zelensky's notion of a 'just peace' and what that looks like, what are the components of that, and how do we help Ukraine get to that point," Kirby said.

"Mr. Putin it's obviously not interested in diplomacy right now," he said. "Quite the contrary. He's interested in killing more Ukrainian civilians," he added.

According to him, Zelenskyy was aware of the risks he was taking in leaving Ukraine, and the US was working to ensure the trip is safe.

"We are working in lockstep with him and his staff to make sure we can make this trip safely, both coming through the United States, and coming back. We will continue to support his travel requirements as best we can," he said.

Kirby declined to say when the sophisticated Patriot missile defence system that Biden is expected to announce Wednesday would be operational inside Ukraine.

"It takes a long time for our troops to get trained on how to operate them. So, we will have to factor all of that in," he said.

On the morning of December 21, the Office of the President of Ukraine officially confirmed Zelenskyy's visit to the United States.

A representative of the White House said that US President Joe Biden will announce a new significant assistance package to Ukraine worth about $2 billion, which will include a battery of Patriot air defence systems.

US President Joe Biden will meet Zelenskyy at the White House at 21:00 Kyiv time.