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, Reuters reported.

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.

Washington has provided tens of billions of dollars worth of U.S. military and economic aid to Ukraine since it was invaded by Russia in February of 2022, funding that Trump has repeatedly criticized and rallied against with other Republican lawmakers, read the report.

Trump insisted last year that Putin never would have invaded Ukraine if he had been in the White House at the time. He told Reuters Ukraine may have to cede territory to reach a peace agreement, something the Ukrainians reject and Biden has never suggested.

Zelenskiy said on Thursday he was not aware of any details of Trump’s plan to end the Ukraine war quickly and that he was convinced a rapid end would entail major concessions for Kyiv.

According to the Government Accountability Office, Congress appropriated over $174 billion to Ukraine under Biden. The pace of the aid is almost sure to drop under Trump with Republicans set to take control of the U.S. Senate with a 52-seat majority.

Control of the U.S. House of Representatives in the next Congress is not yet clear with some votes still being counted. Republicans have won 213 seats, according to Edison Research, just shy of the 218 needed for a majority. If Republicans win both chambers, it will mean the majority of Trump’s agenda will have a significantly easier time passing through Congress, Reuters reported.

Republican U.S. Senator Bill Hagerty, a Trump ally who is considered a top contender for secretary of state, criticized U.S. funding for Ukraine in a CBS interview.

“The American people want sovereignty protected here in America before we spend our funds and resources protecting the sovereignty of another nation,” Hagerty said.

The 2-1/2-year-old war in Ukraine is entering what some officials say could be its final act after Moscow’s forces advanced at the fastest pace since the early days of the war.

Any fresh attempt to end the war is likely to involve peace talks of some kind, which have not been held since the early months of the war.

Moscow’s forces occupy around a fifth of Ukraine. Russia says the war cannot end until its claimed annexations are recognized. Kyiv demands all of its territory back, a position that has largely been supported by Western allies.