Donald Trump lashes out at CIA boss John Brennan and questions whether he was behind ‘dirty dossier’ leak after spy chief warned his tweeting was ‘dangerous’

DONALD Trump has pointed the finger at outgoing CIA chief John Brennan by asking whether he was responsible for leaking news of the Russian dossier.

The President-elect insinuations come after Brennan told the politician to watch what he says by hinting he doesn't understand the real threat posed by Vladimir Putin and Moscow.

But Trump quickly hit back on social media by tweeting: 'Was this the leaker of Fake News?' - a reference to his beliefs that someone from the security services was responsible for telling the press the Russians had a 'dirty dossier' on him.

Earlier, the CIA director said in an interview with Fox News that Trump lacks 'full appreciation and understanding' of the implications of lifting sanctions on Russia.

Trump then tweeted back he 'couldn't do much worse' as president.

'"Outoing CIA Chief, John Brennan, blasts Pres-Elect Trump on Russia Threat. Does not fully understand." Oh really, couldn't do much worse - just look at Syria (red line), Crimea, Ukraine and the build-up of Russian nukes. Not good! Was this the leaker of Fake News?' Trump tweeted.

Brennan turned on Trump in a revealing interview on Fox News Sunday.

"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," Brennan said.

"Now that he's going to have an opportunity to do something for our national security as opposed to talking and , he's going to have tremendous responsibility to make sure that US and national security interests are protected."

The outgoing CIA director also highlighted the bitter state of Trump's relations with American intelligence agencies.

Brennan's pointed message came just five days before Trump becomes the nation's 45th president amid lingering questions about Russia's role in the 2016 election even as the focus shifts to the challenges of governing.

"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," Brennan said.

"Spontaneity is not something that protects national security interests," Brennan declared.

Trump remained behind closed doors in his Manhattan high rise Sunday.

His team worked to answer questions about his plans at home and abroad once he's sworn into office on Friday.

Among Trump's immediate challenges: the United States' complicated relationship with Russia, crafting an affordable health care alternative that doesn't strip coverage from millions of Americans, and growing questions about the legitimacy of his presidency.

Without providing details, Trump promised his plan to replace the nation's health care law would provide universal coverage, according to a Washington Post interview published late Sunday.

"We're going to have insurance for everybody," he said. "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."

Retired Gen. Michael Flynn, who is set to become 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, Trump's team publicly acknowledged the conversations on Sunday.

"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 Vice President-elect Mike Pence.

Repeated contacts just as Obama imposed sanctions have even raised questions about whether Trump's team discussed or even helped shape Russia's response.

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

Trump has repeatedly called for a better relationship between the US and Putin's government.

He even suggested in an interview with the Wall Street Journal on Friday that he'd consider easing the latest sanctions on Russia.