The Kremlin on Monday denied reports that U.S. President-elect Donald Trump had spoken to Russian President Vladimir Putin in recent days, and said Putin had no concrete plans yet to speak to Trump.

The Washington Post first reported that the call had taken place, citing unidentified sources, and said that Trump had told Putin that he should not escalate the Ukraine war. Reuters also reported on the call, citing an unidentified source it said was familiar with the conversation.

"This is completely untrue. This is pure fiction, it's just false information," Kremlin spokesman Dmitry Peskov told reporters. "There was no conversation." 

"This is the most obvious example of the quality of the information that is being published now, sometimes even in fairly reputable publications," Peskov said. 

Asked if Putin had plans for any contacts with Trump, Peskov said: "There are no concrete plans yet."

The 2-½-year-old in Ukraine is entering what some officials say could be its final - and most dangerous - phase as Moscow's forces advance at their fastest pace since the early weeks of the conflict.

Trump said during campaigning that he could bring peace in Ukraine within 24 hours if elected, but has given few details on how he would seek to end the biggest land war in Europe since World War Two.

Putin congratulated Trump on Thursday, praised him for showing courage when a gunman tried to assassinate him, and said Moscow was ready for dialogue with the Republican president-elect.

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.

Massive drone attack on Moscow

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.

Russia has been signaling to the United States and its allies for weeks that if they give permission to Ukraine to strike deep into Russian territory with Western-supplied missiles, then Moscow will consider it a major escalation.

Putin said on Sept. 12 that Western approval for such a step would mean "the direct involvement of NATO countries, the United States and European countries in the war in Ukraine" because NATO military infrastructure and personnel would have to be involved in the targeting and firing of the missiles. 

The Kremlin said that it sensed European powers were nervous about Trump's election, but that they were still "pumping weapons into Ukraine in order to continue this war to the end."

Peskov was asked about reports that British Prime Minister Keir Starmer and French President Emmanuel Macron wanted to convince U.S. President Joe Biden to allow Ukraine to fire Storm Shadow missiles deep into Russia.

WATCH | Trump has criticized sending Ukraine billions in military aid: 

Ukraine, Russia brace for Trump’s return to the White House

Some Ukrainians are expressing uneasiness that Donald Trump’s U.S. election win could mean an end to the country's support against Russia. Trump has publicly criticized sending Ukraine billions in military aid, and has said he could make a deal to ‘end the war in a day.’

"Nothing can be ruled out," Peskov said, adding that European leaders still wanted to inflict a strategic defeat on 
Russia. 

"We, in turn, will continue the SMO (special military operation) until we achieve all our goals," Peskov said. "And it is probably important to note that no individual types of weapons are able to change this dynamic anymore."

According to the Government Accountability Office, Congress appropriated over $174 billion US to Ukraine under Biden, funding that Trump has repeatedly criticized and rallied against with other Republican lawmakers.

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.

Zelensky 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.

WATCH | Expert weighs in on Trump, Ukraine and Musk: 

Musk will be a ‘force’ in U.S. government, war in Ukraine: expert says | Canada Tonight

Ukrainian President Volodymyr Zelenskyy said on Wednesday that he spoke with U.S. president-elect Donald Trump following his victory. Additionally, multiple outlets have reported that billionaire Elon Musk joined the call as well. Nicholas Drummond, a former British Army officer, says Musk could be a ‘good’ influence on Trump.

Putin, who ordered thousands of troops into Ukraine in 2022 after eight years of fighting in eastern Ukraine, casts the war as a battle between Russia and the declining West, which he says ignored Russia's interests after the 1991 Soviet collapse.

Ukraine and its Western allies say Putin unleashed an imperial-style war against its smaller neighbour and have repeatedly said that if Russia wins the war then autocratic countries across the world will be emboldened. 

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.