(CNN) Former Director of National Intelligence James Clapper said Friday night that the intelligence community's assessment of Russian interference in the 2016 presidential race "cast doubt" on the legitimacy of President Donald Trump's victory.

"Our intelligence community assessment did, I think, serve to cast doubt on the legitimacy of his victory in the election," Clapper said on CNN's "Erin Burnett Out Front."

Clapper said he disclosed the information to the President during a briefing on the assessment in January, and that particular aspect seemed to concern Trump the most.

"I think that transcends, unfortunately, the real concern here, which is Russian interference in our political process, which by the way is going to continue," Clapper said.

The revelation came hours after the President tweeted about the "Russian hoax," writing "the greatest influence over our election was the Fake News Media 'screaming' for Crooked Hillary Clinton." At a rally in Alabama later Friday evening, Trump doubled down, saying, "No, Russia did not help me, that I can tell you, OK?"

The agencies of the intelligence community have overwhelmingly said they believe Russian intelligence attempted to influence the US election. At a May hearing, Clapper said that a report concluding that Russia meddled in the US election was prepared by three agencies -- the CIA, FBI and National Security Agency -- and managed by Clapper's office, which oversees the entire intelligence community.

At another May hearing, Virginia Sen. Mark Warner, the top Democrat on the Senate intelligence committee, asked six intelligence leaders — including Trump's director of National Intelligence, Dan Coats, and CIA Director Mike Pompeo — whether they agreed that Russian intelligence agencies were responsible for the hacking and leaking of information to influence the US election.

All six said yes.

Although Trump has publicly cast doubt on the veracity of the intelligence community's findings about Russian interference, Clapper said Trump was "very solicitous, courteous, even complimentary" during the private briefing on the findings.

"And he did listen," Clapper added.

"I thought we successfully conveyed the message, because the substantiating evidence was quite compelling," Clapper said.