Allegations of Russian interference in US politics cast a shadow over inaugural preparations on Sunday, with the CIA chief warning that the president-elect must tamp down his shoot-from-the-hip style to protect national security.
John Brennan's warning to the incoming Republican president -- just days before Friday's inauguration -- came as US senators launched a bipartisan probe into Moscow's alleged meddling in the 2016 election.
"I don't think he has a full appreciation of Russian capabilities, Russia's intentions and actions," Brennan said of Trump on Fox News Sunday.
"I think Mr. Trump has to be very disciplined in terms of what it is that he says publicly -- he is going to be, in a few days' time, the most powerful person in the world, in terms of sitting on top of the United States government and I think he has to recognize that his words do have impact."
"What I think Mr. Trump has to understand is that this is more than being about him, it's about the United States and national security... he's going to have the opportunity to do something for national security as opposed to talking and tweeting," Brennan said.
"Spontaneity is not something that protects national security interests, so therefore, when he speaks, when he reacts, he has to make sure he understands that the implications and impact on the United States could be profound."
US intelligence agencies allege that Russian President Vladimir Putin ordered a covert effort to interfere in the election to boost Trump, 70, and harm his opponent, Democrat Hillary Clinton.
A report from the Director of National Intelligence released this month said hackers working for Russia penetrated Democratic Party computers and accounts to release files embarrassing to Clinton, and also conducted a campaign of media manipulation with the same aim.
The investigation, backed by both Democrats and Republicans on the Senate Intelligence Committee, could trigger forced testimony by officials of both Barack Obama's outgoing administration and the incoming Trump government.
Speculation was also swirling around an unproven dossier compiled by a former British MI6 intelligence agent, which alleged close ties between the Trump campaign and Russian government, and said Moscow had compromising video of Trump.
The fact that intelligence agencies had briefed the president-elect on the dossier -- which was later published online by BuzzFeed -- lent the allegations credence, but Brennan said the intelligence community was only "making sure that the president-elect was aware that it was circulating."
"I think there are some very salacious allegations in there -- again, unsubstantiated," he said, saying it was "a responsibility in the minds of the intelligence directors" to inform Trump as well as the Obama White House of the report.