The CIA's outgoing director has said Donald Trump lacks a full understanding of the threat Moscow poses to the United States.

John Brennan delivered a public lecture that infuriated the president-elect and further highlighted his bitter relations with American intelligence agencies.

His pointed message on national television came just five days before Mr Trump becomes the nation's 45th president amid lingering questions about Russia's role in the 2016 election.

"Now that he's going to have an opportunity to do something for our national security as opposed to talking and tweeting, he's going to have tremendous responsibility to make sure that US and national security interests are protected," Mr Brennan said on Fox News on Sunday, warning that the president-elect's impulsivity could be dangerous.

"Spontaneity is not something that protects national security interests," he said.

Mr Trump has repeatedly called for a better relationship between the US and Vladimir Putin's government and suggested in an interview with the Wall Street Journal on Friday that he would consider easing the latest sanctions on Russia.

But Mr Brennan said: "I think he has to be mindful that he does not have a full appreciation and understanding of what the implications are of going down that road."

Mr Trump, who has unleashed a series of aggressive tweets against the US intelligence community and his political rivals in recent weeks, responded aggressively on Twitter several hours later.

"Was this the leaker of Fake News?" he tweeted, referring to a recent document that contains unverified financial and personal information that could be damaging to the President-elect.

Mr Trump remained in his Manhattan high-rise on Sunday, as his team worked to answer questions about his plans at home and abroad once he is sworn into office on Friday.

CIA Director John Brennan has stern warning for Donald Trump

His immediate challenges include the United States' complicated relationship with Russia, an affordable health care alternative that does not strip coverage from millions of Americans, and growing questions about the legitimacy of his presidency.

Without providing details, Mr Trump promised his plan to replace the nation's health care law would provide universal coverage.

"We're going to have insurance for everybody," he said in a Washington Post interview.

"There was a philosophy in some circles that if you can't pay for it, you don't get it. That's not going to happen with us."

Meanwhile, civil rights legend and congressman John Lewis is among several Democrats who vowed to skip Mr Trump's inauguration, saying that Russian interference in the 2016 election delegitimises his presidency.

"There will be many more members who join us in this decision," California Democrat Jared Huffman wrote on his Facebook page.

Mr Trump's lieutenants pushed back hard on Sunday in a round of television interviews.

"I think it's incredibly disappointing and I think it's irresponsible for people like himself to question the legitimacy of the next United States president," incoming White House chief of staff Reince Priebus said of Mr Lewis on ABC's This Week.

Mr Priebus insisted that Republicans did not question Barack Obama's legitimacy, but later acknowledged that conservatives, led by Mr Trump himself, spent years questioning Mr Obama's eligibility to serve as president, suggesting he was not born in the United States.

Vice president-elect Mike Pence told Fox News on Sunday that he hoped Mr Lewis would change his mind and attend the inauguration.

Mr Trump has done little to encourage unity in recent days, instead inflaming tensions with his critics through a series of tweets.

The incoming president tweeted on Saturday that Mr Lewis should pay more attention to his "crime ridden" Atlanta-area district, adding that the civil rights leader was "all talk".

Mr Lewis suffered a fractured skull when he led a march in Selma, Alabama, more than 50 years ago and has devoted his life to civil rights.

Current White House chief of staff Denis McDonough said Mr Lewis "has literally fought, bled and gone to jail" during what he called his "remarkable life" and encouraged the incoming president to move past his criticism.

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"That would be the kind of thing that would not only send a message to the American people that we're prepared to work together, but would also send a message to the Russians that we are united," Mr McDonough told CNN's State Of The Union.

Questions about Mr Trump's relationship with Russia have dominated the days leading up to his inauguration.

Retired general Michael Flynn, who is set to become Mr Trump's national security adviser, has been in frequent contact with Russia's ambassador to the US in recent weeks, including on the day the Obama administration hit Moscow with sanctions in retaliation for the alleged election hacking, a senior US official said.

After initially denying the contact took place, Mr Trump's team later acknowledged the conversations.

"The conversations that took place at that time were not in any way related to the new US sanctions against Russia or the expulsion of diplomats," said Mr Pence on Fox News on Sunday.

Repeated contacts just as Mr Obama imposed sanctions would raise questions about whether Mr Trump's team discussed - or even helped shape - Russia's response.

Mr Putin, unexpectedly, did not retaliate against the US for the sanctions or the expulsion of 35 Russian diplomats, a decision Mr Trump quickly praised.

Meanwhile, US vice-president Joe Biden, on a last foreign trip before leaving office, has met with Ukraine's president and called on the impending Trump administration to retain Ukraine-related sanctions against Russia.

Mr Biden's comments at a briefing with President Petro Poroshenko came after Mr Trump indicated in a Times interview that he could end sanctions imposed in the aftermath of Russia's 2014 annexation of Crimea in return for a nuclear arms reduction deal.

"The Crimea-related sanctions against Russia must remain in place until Russia returns full control to the people of Ukraine," Mr Biden said.

Other US sanctions are connected to Russia's involvement in the separatist war in eastern Ukraine. Mr Biden said that Russia must fulfil its obligations under the 2015 Minsk agreement on ending that conflict.