Donald Trump has lashed out at outgoing CIA chief John Brennan, and questioned whether he was responsible for leaking the Russian dossier.

Brennan had earlier told the president-elect to watch what he says, suggesting he doesn't understand the threats posed by Moscow.

But Trump fired back on Twitter on Sunday night. He wrote: 'Was this the leaker of Fake News?'

The jab was a reference to a recent intelligence briefing that raised questions about Trump's connections to Russian President Vladimir Putin.

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John Brennan said on Fox News Sunday that Donald Trump's 'talking and tweeting' is not in the nation's interest and the president-elect lacks a full understanding of the threat Russia poses

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

Trump acknowledged Brennan's criticism and tweeted that 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 unleashed on Trump in a scathing interview on Fox News Sunday - a show the president-elect routinely watches.

'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 tweeting, he's going to have tremendous responsibility to make sure that U.S. and national security interests are protected.'

Donald Trump has lashed out at outgoing CIA chief John Brennan hinting he may have been behind the leaking of a Russian dossier

Brennan had earlier ripped into Trump on Fox News warning him to watch what he says and suggesting the president-elect doesn't understand the threats posed by Russia

Trump's transition team has not yet commented.

The extraordinary televised lecture to the incoming president highlighted the bitter state of Trump's relationship with the American intelligence community just days before he is inaugurated as the nation's 45th president.

Trump has repeatedly shrugged off intelligence that's convinced Republicans and Democrats that Russia tried to help him win election.

He's publicly called for a better relationship between the U.S. and President Vladimir Putin's government, and suggested in an interview with the Wall Street Journal on Friday that he'd consider easing sanctions imposed by President Barack Obama as payback for the alleged election hacking.

Trump also has suggested that the intelligence community is out to get him — including by the leak of a document containing potentially damaging, but unverified, financial and personal information on Trump.

Trump has likened the situation to 'Nazi Germany.' Brennan on Sunday called that comparison 'outrageous' and said the intelligence community wanted the president-elect to know that the document was circulating among some news outlets.

But an array of revelations has shed more light on the Trump-Putin relationship. Ret. Gen. Michael Flynn, who is set to become Trump's national security adviser, and Russia's ambassador to the U.S. have been in frequent contact in recent weeks, including on the day the Obama administration hit Moscow with sanctions in retaliation for the alleged election hacking, a senior U.S. official says.

After initially denying that Flynn and Ambassador Sergey Kislyak spoke Dec. 29, a Trump official said late Friday that the transition team was aware of one call on the day the Obama administration imposed the sanctions.

Vice President-elect Mike Pence, also in an appearance on 'Fox News Sunday,' denied that Flynn and Kislyak discussed anything relating to the sanctions.

Pence said he talked to Flynn about this on Saturday. 'The conversations that took place at that time were not in any way related to the new U.S. sanctions against Russia or the expulsion of diplomats.'

Repeated contacts just as Obama imposed sanctions would raise questions about whether Trump's team discussed — or even helped shape — Russia's response. Russian President Vladimir Putin unexpectedly did not retaliate against the U.S. for the sanctions or the expulsion of 35 Russian diplomats, a decision Trump quickly praised.

Pence on Sunday also denied that anyone from the Trump campaign contacted Russian officials before the election.

Brennan on Sunday roundly denounced Trump's approach to Russia and other national security threats, suggesting the president-elect has much to understand before he can make informed decisions on such matters.

Trump's impulsivity could be dangerous, Brennan suggested.

'Spontaneity is not something that protects national security interests,' Brennan said, saying twice that the matter is 'more than being about him.'

'I think Mr. Trump has to understand that absolving Russia of various actions it has taken in the past number of years is a road that he needs to be very, very careful about moving down.'

There are costs to casting doubts on the credibility of the national intelligence community, Brennan added.

'The world is watching now what Trump says and listening very carefully. If he doesn't have confidence in the intelligence community, what signal does that send to our partners and allies as well as our adversaries?' Brennan said.