US Vice-President Joe Biden has told Donald Trump to "grow up" and criticised his “absolutely mindless” attacks on intelligence agencies.
Mr Biden spoke after Mr Trump cast more doubt on allegations Russia meddled in the US presidential election, with a series of tweets.
Mr Trump's tweets come before he is due to be briefed on a report by intelligence agencies about the hacking allegations, on Friday.
An unclassified version of the report will be made public next week.
But already downplaying the report, Mr Trump questioned why the FBI was denies access to certain computer servers.
The Democratic National Committee would not allow the FBI to study or see its computer info after it was supposedly hacked by Russia......
— Donald J. Trump (@realDonaldTrump) January 6, 2017
So how and why are they so sure about hacking if they never even requested an examination of the computer servers? What is going on?
— Donald J. Trump (@realDonaldTrump) January 6, 2017
Mr Biden hit out at Mr Trump for failing to acknowledge the expertise of the intelligence community.
"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."
Responding to Mr Trump's Twitter attacks on Democrats and others, Mr Biden added: "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".
Hacking claims
Director of national intelligence James Clapper said he had a very high level of confidence that Russia hacked Democratic Party emails.
He said Russia - acting on orders from Vladimir Putin - helped spread propaganda and fake news to help Donald Trump's victory.
"We assess that only Russia's senior-most officials could have authorised the recent election-focused data thefts and disclosures," Mr Clapper said.
Mr Clapper claimed Russia has a "long history" of interfering in elections going back to the 1960s.
But he said they had never been as "aggressive" and "direct" as this time and pointed to a "highly-advanced offensive cyber programme".
Asked by a congressman on the Senate Armed Services Committee if they "will ascribe a motivation to Putin", Mr Clapper said: "Yes".
Committee chairman Senator John McCain said "every American should be alarmed" by Russia's meddling.
But he said the committee's probe did not intend to question the result of November's election.
The alleged hacking led to new sanctions by President Barack Obama against Moscow and 35 Russian diplomats were expelled from the US.
Moscow has denied the hacking allegations and dismissed the sanctions as an attempt to "harm Russian-American ties."
The report on election meddling and other foreign countries has already been delivered to Mr Obama.
