United States Vice-President Joe Biden has issued a stern warning to President-elect Donald Trump, telling him to "grow up" and "be an adult" in regard to his attacks on the US intelligence community.

Key points: Trump has repeatedly cast doubt over US intelligence claims that Russia staged cyber attacks

Trump has repeatedly cast doubt over US intelligence claims that Russia staged cyber attacks Biden says the behaviour is "dangerous" but assumes Mr Trump's behaviour will change on Jan 20th

Biden says the behaviour is "dangerous" but assumes Mr Trump's behaviour will change on Jan 20th Mr Trump is due to be officially briefed on the cyber attacks

Top US intelligence official James Clapper has told Congress he is "even more resolute" in his belief that Russia staged cyber attacks on Democrats in the 2016 election campaign.

But Mr Trump has repeatedly taken to social media to reject the allegations.

It comes as the US' top intelligence officials prepare to make their most detailed and persuasive cast yet to the President-elect that Russia interfered with the election.

In an interview with PBS NewsHour, Mr Biden said it was time for Mr Trump "to be an adult".

"You're President. You've got to do something. Show us what you have," Mr Biden said.

Mr Biden said it was "dangerous" for the President-elect to publicly criticise the US intelligence community and "absolutely mindless" for a President not to have confidence in or listen to the agencies.

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He said it would be legitimate to question intelligence and ask for more detail or disagree, but that Mr Trump was claiming he knew more than the experts.

Mr Biden described Mr Trump's comments as akin to saying: "I know more about physics than my professor".

Striking a notably more positive tone about the incoming president, Mr Biden remarked that Mr Trump was surrounding himself with "very smart people" like retired Marine Corps General James Mattis, the billionaire businessman's pick for Defence Secretary.

He said he assumed Mr Trump's behaviour would change once he entered office on January 20.

Asked at a press briefing on Thursday whether he had full confidence in US intelligence agencies, now-Speaker Paul Ryan offered a tepid endorsement of the intelligence agencies that began and ended on a note of criticism.

"Well, they don't always get everything right. We've seen that clearly," Mr Ryan said.

"But I do have faith that our men and women in our intelligence community are doing an incredible job, sacrificing for our country. But there's always room for improvement."

US Intelligence 'resolute' on Russian involvement

Mr Trump will be briefed by intelligence agency chiefs on Friday (local time) on the hacks.

In his address at a hearing of the Senate Armed Services Committee on Thursday, Mr Clapper said he had a very high level of confidence that Russia hacked Democratic Party institutions and operatives.

"Our assessment now is even more resolute than it was on October 7 when the Government first publicly accused Russia," Mr Clapper said.

Sorry, this video has expired The head of US intelligence says Russia's hacking program poses a major threat to US interests.

Mr Clapper, CIA director John Brennan and FBI director James Comey are preparing to point to multiple motives for Moscow's alleged meddling when they brief Mr Trump.

President Barack Obama received a briefing on Thursday.



An unclassified version of the intelligence review of Russian interference in the election would be made public early next week and would assign a motive for the attacks, Mr Clapper said.

The Washington Post, citing anonymous officials, reported on Thursday that intelligence agencies have identified parties who delivered stolen Democratic emails to WikiLeaks.

The officials also said there were disparities between efforts to infiltrate Democratic and Republican networks, and said the US intercepted communications in which Russian officials celebrated Mr Trump's victory.

It was not clear which of those details were included in the classified report.

Sean Spicer, whom Mr Trump has chosen as White House communications director and press secretary, said the billionaire will go into Friday's meeting "prepared to listen and understand how they got to the conclusions".

Asked on ABC's "Good Morning America" if Mr Trump would have an open mind, Mr Spicer replied, "100 percent".

"The President-elect, I think, has a healthy scepticism on everything," the spokesman said, adding that "a rush to judgment is not in the country's best interest".

"I think the idea that he's approaching this in a very logical, very methodical way … is the right way to go about it."

US money may be used to pay for Mexico border wall

Mr Spicer also addressed continued questions over who would pay for Mr Trump's touted border wall between the US and southern neighbour Mexico.

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Mr Trump tweeted on Friday that Mexico would reimburse American taxpayers for a new border wall and that US money spent would be for the "sake of speed".

It came as top aides considered a plan to ask Congress to ensure money is available in US coffers for the wall, but to rely on existing law that already authorises fencing and other technology along the southern border.

The Associated Press reported the potential approach was confirmed on Thursday by two congressional officials and a senior transition official with knowledge of the discussions, all of whom spoke on condition of anonymity.

Mr Spicer said putting US money up-front "doesn't mean he's broken his promise".

"I think he's going to continue to talk to them [the Mexican Government] about that," he said.

AP/ABC