WASHINGTON — 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 effect declared war on the intelligence agencies in a statement issued by his transition team Friday evening.

“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 antagonizing the agencies, to carry out priorities such as combating terrorism 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 Hayden, who was the director of the NSA and later the CIA under President George W. Bush.

With the partisan emotions on both sides — Trump’s supporters see a plot to undermine his presidency, and Hillary Clinton’s supporters see a conspiracy to keep her from the presidency — the result is an environment in which even those basic facts become the basis for dispute.

Trump’s team lashed out at the agencies after The Washington Post reported that the CIA believed that Russia had intervened to undercut 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.

The president-elect finds himself in a bind after strenuously rejecting for months all assertions that Russia was working to help him.

While there is no evidence that the Russian efforts 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, including the idea that Russian President Vladimir Putin schemed to put him in office.

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.

They say that Russia was behind the election meddling, but that the scope and intent of the operation need deep investigation, hearings, and public reports.

One question they may want to explore is why the intelligence agencies believe that the Republican networks were compromised while the FBI, which leads domestic cyberinvestigations, has apparently told Republicans that it has not seen evidence of that breach.

Senior officials say the intelligence agencies’ conclusions are not being widely shared, even with law enforcement.

“We cannot allow foreign governments to interfere in our democracy,’’ said Representative Michael McCaul, a Texas Republican who is the chairman of the Homeland Security Committee. “When they do, we must respond forcefully, publicly, and decisively.’’

He has promised hearings, saying the Russian activity was “a call to action,’’ as has Senator John McCain, one of the few senators left from the Cold War era, when the Republican Party made opposition to the Soviet Union — and later deep suspicion of Russia — the centerpiece of its foreign policy.

Even one of Trump’s most enthusiastic supporters, Representative Devin Nunes of California, said Friday that he had no doubt about Russia’s culpability. His complaint was with the intelligence agencies, which he said had “repeatedly’’ failed “to anticipate Putin’s hostile actions,’’ and with the Obama administration’s lack of a punitive response.