CIA Director John Brennan on Sunday had a stern parting message for Republican Donald Trump days before he assumes the US presidency, cautioning him against loosening sanctions on Russia and warning him to watch what he says.

Brennan rebuked the president-elect for comparing US intelligence practices to Nazi Germany in comments that laid bare the friction between Trump and the intelligence community he has criticised and is on the verge of commanding.

In an interview with Fox News Sunday, Brennan questioned the message sent to the world if the president-elect broadcasts that he does not have confidence in the United States' own intelligence agencies.

"What I do find outrageous is equating intelligence community with Nazi Germany," Brennan said. "I do take great umbrage at that."

His comments followed a tumultuous week of finger-pointing between Trump and intelligence agency leaders over an unsubstantiated report of compromising information Moscow had collected on Trump.

The unverified dossier was summarised in a US intelligence report presented to President Barack Obama and Trump this month that concluded Russia tried to sway the outcome of the Nov. 8 election by hacking and other means. It did not make an assessment on whether the attempts were successful.

Trump accused the intelligence community of leaking the compromising information, which its leaders denied. They said it was their responsibility to inform the president-elect that the allegations were being circulated.

Brennan sounded an alarm on the topic of US relations with Russia, something Trump has vowed to improve as he beats back criticism that he is too eager to make an ally of Russian President Vladimir Putin.

Trump does not yet have a full understanding of Russia's actions, Brennan said, noting its seizure of Crimea from Ukraine, its support for President Bashar al-Assad in Syria's civil war and Moscow's activities in the cyber realm.

"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 I think needs to be very, very careful about moving down," he told Fox.

In an interview with the Wall Street Journal published on Friday, Trump suggested he might do away with sanctions imposed by the Obama administration in late December in response to alleged cyber attacks if Moscow proves helpful in battling terrorists and reaching other US goals.

Russian relations

The president-elect's comments about Putin and his reluctance to assign blame to Moscow for the hacking of Democratic political groups has opened him up to criticism that he will be too soft on Russia.

For months, Trump had publicly doubted US intelligence conclusions about the cyber attacks before acknowledging at a news conference on Wednesday that he thought Russia was behind the hacking.

Vice President-elect Mike Pence told the Fox program, "What the president-elect is determined to do is to explore the possibility of better relations."

Pence did not say whether Trump would undo some of the sanctions and diplomatic expulsions Obama had slapped on Moscow.

He confirmed Trump's incoming national security adviser, Michael Flynn, held conversations with the Russian ambassador around the time the sanctions were imposed, but said the talks "were not in any way related to the new US sanctions against Russia or the expulsion of diplomats."

However, Pence denied that Trump's political team had any contact with Russian officials during the presidential campaign. "Of course not," he told Fox.

Leaders of the Senate Intelligence Committee said on Friday they will investigate alleged Russian attempts to influence the election and any links between Russia and the political campaigns.

Brennan also on Sunday said Trump needs to be mindful about his off-the-cuff remarks once he takes the oath of office on Friday, alluding to his penchant for making broad pronouncements on Twitter.

"Spontaneity is not something that protects national security interests," Brennan said. "So therefore when he speaks or when he reacts, just make sure he understands that the implications and impact on the United States could be profound.

"It's more than just about Mr. Trump. It's about the United States of America."