US Vice-President Joe Biden has told President-elect Donald Trump to "grow up" and criticised his attacks on the intelligence community.
On Friday, Mr Trump will be briefed on allegations that Russia meddled in the US presidential election - claims he has cast doubt on.
Mr Biden said it was "absolutely mindless" for the president-elect not to have faith in intelligence agencies.
Russia denies hacking alleged to have helped Mr Trump defeat Hillary Clinton.
Late on Thursday, Mr Trump questioned how intelligence agencies were confident about the alleged Russian hacking "if they never even requested an examination of the computer servers" belonging to the Democratic National Committee (DNC).
What Mr Biden said
"For a president not to have confidence in, not to be prepared to listen to, the myriad intelligence agencies, from defence intelligence to the CIA, is absolutely mindless," he said in an interview with the PBS network.
"The idea that you may know more than the intelligence community knows - it's like saying I know more about physics than my professor. I didn't read the book, I just know I know more."
When asked what he thought of Mr Trump's regular attacks on Twitter, Mr Biden said: "Grow up Donald, grow up, time to be an adult, you're president. Time to do something. Show us what you have."
He went on to call Mr Trump "a good man".
What the report says about hacking allegations
Mr Biden said he had read a US intelligence agencies report outlining Russian involvement, the details of which are starting to emerge in US media.
US authorities had also identified Russian actors who delivered stolen Democratic emails to the Wikileaks website.
NBC News says the alleged Russian hacking targeted not just the DNC but also the White House, joint chiefs of staff, the department of state and large US corporations.
An unclassified version will be made public next week.
Mr Biden said the report clearly details "that the Russians did, as a matter of policy, attempt to affect and... discredit the US electoral process".
He said the hacking was part of a systematic campaign to undermine Mrs Clinton. Her campaign manager, John Podesta, was among those hacked on the DNC server.
On Thursday, the Director of National Intelligence, Gen James Clapper, told a hearing of the Senate Armed Services Committee that Russian President Vladimir Putin ordered the hack, and said the motive would be revealed next week.
President Barack Obama last week ordered the expulsion of 35 Russian diplomats from the US over the alleged hacking. Russia has said it will not reciprocate.
What Mr Trump says
Mr Trump has repeatedly rejected allegations that the Russian government hacked into the computers of Mr Podesta or the servers of the Democratic National Committee.
On Wednesday, he repeated a suggestion that "a 14-year-old" may have been responsible for the breach.
On Thursday, he said he was a "big fan" of intelligence agencies, after months of casting doubt on the Russian link, but later went on to raise questions over how the Democratic Party responded to the security breach.
"How and why are they so sure about hacking if they never even requested an examination of the computer servers? What is going on?" he asked in a tweet.
Last week, Mr Trump said he would announce information about hacking "on Tuesday or Wednesday", but no announcement came.
Mr Trump's inauguration will take place on 20 January.