President-elect Donald Trump recently spoke with Russian President Vladimir Putin, advising him to avoid further escalation in the Ukraine war, according to a source familiar with the call.

According to Reuters, Trump also spoke with Ukrainian President Volodymyr Zelenskiy on Wednesday. Ukraine’s foreign ministry reported it had not been informed beforehand about the Trump-Putin call and could not offer a position on it.

Meanwhile, President Joe Biden, is preparing to lobby Trump against abandoning U.S. support for Kyiv.

Since Russia’s February 2022 invasion, the U.S. has provided substantial military and economic aid to Ukraine, amounting to $174 billion, funding that Trump and other Republican lawmakers have consistently opposed. Trump has asserted that Putin would not have invaded had he been president, and he has suggested Ukraine might need to cede some territory for peace—something Kyiv and Biden both reject. The President-elect has pledged to end the conflict swiftly once he takes office, without specifying, however, how he’ll do it.

Following his Nov. 5 election victory over Vice President Kamala Harris, Trump is set to take office on Jan. 20. The White House has confirmed Biden’s invitation for Trump to visit the Oval Office on Wednesday. National Security Adviser Jake Sullivan emphasized on Sunday that Biden’s main focus will be ensuring a peaceful transfer of power. He also plans to discuss ongoing U.S. interests in Europe, Asia, and the Middle East with the president-elect.

Biden, Sullivan told CBS News, will make a case for maintaining U.S. support for Ukraine, cautioning that withdrawing aid could lead to greater instability in Europe.

Zelenskiy, speaking at a news conference at the European Political Community summit in Budapest on Thursday, said that he remains unaware of the specifics of Trump’s plan to end the war swiftly, doubting that such an outcome would be favorable to Ukraine without significant compromises.

“If it’s just fast, it means losses for Ukraine. I just don’t yet understand how this could be in any other way. Maybe we do not know something, do not see,” he noted.