The top U.S. intelligence official said on Thursday he was "even more resolute" in his belief that Russia staged cyber attacks on Democrats during the 2016 election campaign, rebuking persistent scepticism from Republican President-elect Donald Trump about whether Moscow was involved.

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 scepticism ... and disparagement," Clapper said. Vice President-elect Mike Pence has used the expression "healthy scepticism" 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 U.S. 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 defence for intelligence, did not say what made U.S. 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.

MOSCOW DENIES ALLEGATIONS

Moscow denies the allegations. Obama last week ordered the expulsion of 35 Russian suspected spies and imposed sanctions on two Russian intelligence agencies he said were involved in hacking U.S. political groups such as the Democratic National Committee.

In a tweet on Wednesday, Trump was sceptical about a Russian role, criticizing Democrats and praising WikiLeaks founder Julian Assange, whose organisation posted Democrats' emails.

But on Thursday, Trump said in another post on Twitter that he was not against intelligence agencies or in agreement with Assange. "The media lies to make it look like I am against 'intelligence' when in fact I am a big fan!" Trump tweeted.

Clapper said Assange had put American lives in danger and deserved no credibility. McCain and other lawmakers also blasted Assange.

Senator Claire McCaskill, a Democrat, said there would be "howls" from Republicans if a Democrat described intelligence officials as Trump had.

Documents stolen from the DNC and Clinton's campaign chairman, John Podesta, were posted on the Internet before the election, embarrassing her campaign.

U.S. intelligence officials have said Russian cyber attacks were specifically aimed at helping Trump beat Clinton. Several Republicans have acknowledged the Russian hacking but have not linked it to an effort to help Trump win.

Trump and top advisers believe Democrats are trying to delegitimize his victory by accusing Russia of helping him.

Senator Tim Kaine, an Armed Services member who was Clinton's vice presidential running mate, said: "It is my hope that this Congress is willing to stand in a bipartisan way for the integrity of the electoral process."

Graham said Obama's actions against Moscow fell short.

"I think what Obama did was throw a pebble. I'm ready to throw a rock," Graham said. "Putin is up to no good and he better be stopped."