An extraordinary breach has emerged between President-elect Donald Trump and the national security establishment, with Trump mocking US intelligence assessments that Russia interfered in the election on his behalf, and top Republicans vowing investigations into Kremlin activities.

Trump, in a statement issued by his transition team Friday evening, expressed complete disbelief in the intelligence agencies' assessments.

"These are the same people that said Saddam Hussein had weapons of mass destruction," Trump's team said, adding that the election was over and that it was time to "move on".

Though Trump has wasted no time in antagonising the agencies, he will have to rely on them for the sort of espionage activities and analysis that they spend more than $70 billion a year to perform.

At this point in a transition, a president-elect is usually delving into intelligence he has never before seen and learning about CIA and National Security Agency abilities.

But Trump, who has taken intelligence briefings only sporadically, is questioning not only analytic conclusions, but also their underlying facts.

"To have the president-elect of the United States simply reject the fact-based narrative that the intelligence community puts together because it conflicts with his a priori assumptions, wow," said Michael V. Hayden, who was the director of the NSA and later the CIA under President George W. Bush.

Trump's team lashed out at the agencies after The Washington Post reported that the CIA believed that Russia had intervened to undercut Hillary Clinton and lift Trump, and The New York Times reported that Russia had broken into Republican National Committee computer networks just as they had broken into Democratic ones, but had released documents only on the Democrats.

While there is no evidence that the Russian meddling affected the outcome of the election or the legitimacy of the vote, Trump and his aides want to shut the door on any such notion. Instead, Trump casts the issue as an unknowable mystery.

"It could be Russia," he recently told Time magazine. "And it could be China. And it could be some guy in his home in New Jersey."

The Republicans who lead the congressional committees overseeing intelligence, the Pentagon and the Department of Homeland Security take the opposite view.

Rep. Devin Nunes, R-Calif., chairman of the House Intelligence Committee, said Friday that he had no doubt about Russia's culpability.