U.S. President-elect Donald Trump spoke with Russian President Vladimir Putin and advised him not to escalate the Ukraine war, a source familiar with the conversation told Reuters on Sunday, as President Joe Biden plans to urge Trump not to abandon Kyiv.

Trump and Putin spoke in recent days, said the source. Trump spoke with Ukrainian President Volodymyr Zelenskiy on Wednesday. Trump has criticised the scale of U.S. military and financial support for Kyiv, vowing to end the war quickly, without saying how.

Ukraine's foreign ministry said it was not informed in advance of the call between Trump and Putin and subsequently could neither endorse or object to it.

"We do not comment on private calls between President Trump and other world leaders," said Steven Cheung, Trump's communications director, when asked about the phone call, which was first reported by The Washington Post.

The Russian embassy in Washington did not immediately respond to a request for comment.

Republican Trump will take office on Jan. 20 after defeating Vice President Kamala Harris in the Nov. 5 presidential election. Biden has invited Trump to come to the Oval Office on Wednesday, the White House said.

U.S. National Security Adviser Jake Sullivan said on Sunday that Biden's top message will be his commitment to ensure a peaceful transfer of power, and he will also talk to Trump about what's happening in Europe, in Asia and the Middle East.

"President Biden will have the opportunity over the next 70 days to make the case to the Congress and to the incoming administration that the United States should not walk away from Ukraine, that walking away from Ukraine means more instability in Europe," Sullivan told CBS News' "Face the Nation" show.

Sullivan's comments came as Ukraine attacked Moscow on Sunday with at least 34 drones, the biggest drone strike on the Russian capital since the beginning of the war.

When asked if Biden would ask Congress to pass legislation to authorize more funding for Ukraine, Sullivan deferred.

"I'm not here to put forward a specific legislative proposal. President Biden will make the case that we do need ongoing resources for Ukraine beyond the end of his term," Sullivan said.