Director of National Intelligence James Clapper testifies before a Senate Armed Services Committee hearing on foreign cyber threats, on Capitol Hill in Washington, US, January 5, 2017. [Photo/Agencies]

James Clapper, the director of national intelligence, said he had a very high level of confidence that Russia hacked Democratic Party and campaign staff email, and disseminated propaganda and fake news aimed at the Nov. 8 election.

"Our assessment now is even more resolute than it was" on Oct. 7 when the government first publicly accused Russia, Clapper told a hearing of the Senate Armed Services Committee. He said motives for the attack would be made public next week.

Trump on Thursday morning called himself a "big fan" of intelligence agencies. But he has cast doubt on their assessments that Russia targeted the campaign of his opponent, former Secretary of State Hillary Clinton, drawing ire from his fellow Republicans as well as Democrats who are wary of Moscow and distrust Trump's praise of Russian President Vladimir Putin.

The intelligence officials at Thursday's hearing said they worried a lack of support from atop the government could prompt valued staff members to leave their agencies.

"There's a difference between healthy skepticism ... and disparagement," Clapper said. Vice President-elect Mike Pence has used the expression "healthy skepticism" to defend Trump's criticism of intelligence findings.

The hearing was overseen by Republican Senator John McCain, a vociferous Russia critic. It was the first in a promised series of briefings and hearings on allegations that Russia tried to disrupt or influence the US campaign, one of the most bitter in recent history.

McCain told reporters that Senator Lindsey Graham, also a vocal critic of Moscow, would chair a new Armed Services subcommittee dedicated to cyber issues.

"Every American should be alarmed by Russia's attacks on our nation. There is no national security interest more vital to the United States of America than the ability to hold free and fair elections without foreign interference," McCain said.

Trump will be briefed by intelligence agency chiefs on Friday on the hacks. President Barack Obama received a report on the matter on Thursday. An unclassified version will be made public early next week.

"I don't think we've ever encountered a more aggressive or direct campaign to interfere in our election process than we've seen in this case," said Clapper, who leaves when Trump becomes president on Jan. 20. Clapper stopped short of declaring Russia's actions "an act of war," saying that determination was beyond the scope of his office.

Clapper and the two other officials who testified, National Security Agency Director Admiral Mike Rogers, and Marcel Lettre, undersecretary of defense for intelligence, did not say what made US intelligence confident Russia was behind the cyber attacks, a conclusion also reached by several private firms.

Clapper said the hacking did not change any vote tallies.