WASHINGTON -  US spy chiefs said Thursday that the hacking of Democratic Party computers was just one part of a multifaceted campaign by Russia to disrupt the American presidential election.

In a closely-watched Senate hearing, top intelligence officials presented a united front as they reiterated their conclusion that Moscow interfered with last year's campaign and that its cyber-meddling poses a "major threat" to the United States - findings that President-elect Donald Trump has so far refused to accept.

"This was a multifaceted campaign. So the hacking was only one part of it, and it also entailed classical propaganda, disinformation, fake news," Director of National Intelligence James Clapper told the Senate hearing.

While Trump continues to question the evidence of Russian meddling, Clapper, National Security Agency chief Michael Rogers and Marcel Lettre, undersecretary of defence for intelligence, told the committee they are convinced that Moscow is aggressively targeting the United States in offensive cyberspace activities.

"We assess that only Russia's senior-most officials could have authorised the recent election-focused data thefts and disclosures," they said in a joint statement to the Senate Armed Forces Committee.

"Russia is a full-scope cyber actor that poses a major threat to US government, military, diplomatic, commercial and critical infrastructure and key resource networks," they said.

In the first public hearing dealing with Russia's alleged interference since the allegations first came out in October, Clapper said such cyber threats are "challenging public trust and confidence in information, services and institutions."

"Russia has clearly assumed an even more aggressive cyber posture by increasing cyber espionage operations, leaking data stolen from these operations and targeting critical infrastructure systems," he said.

The incoming Republican president has repeatedly cast doubt on the intelligence community's assessment that Moscow was behind the hacking of Democratic Party computers to leak documents and emails that would damage Trump's rival Hillary Clinton in November's election.

But asked in the hearing to provide more proof of their conclusion, Clapper said he could not reveal details publicly.

Trump will be briefed Friday by the heads of the CIA, FBI and DNI on the evidence behind their conclusion on Russia election interference. And a declassified version of a report produced for the White House on the case is expected to be released next week.

The president-elect has raised the ire of the intelligence community by questioning the record of the CIA, FBI and other agencies in providing solid assessments to the government.

Senator John McCain, a senior Republican leader who is the chaiman of the Senate Armed Services Committee, opened the hearing by declaring the Russian interference an "unprecedented attack on our democracy".

"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. 

"That is why Congress must set partisanship aside, follow the facts, and work together to devise comprehensive solutions to deter, defend against, and, when necessary, respond to foreign cyberattacks."

The high stakes Senate Armed Services Committee's hearing comes on a day in which lawmakers are being briefed on the breadth and depth of Russian interference in the American election. It also comes on the eve of President-elect Donald Trump's own in-depth briefing on the matter.

James Clapper, the Director of National Intelligence, told the committee that lawmakers would be briefed next week and a redacted report was scheduled to be released afterwards. "Russia has clearly assumed an even more aggressive cyber posture by increasing cyber espionage operations, leaking data stolen from these operations, and targeting political infrastructures systems," Clapper told the committee. Trump has questioned the intelligence community's findings.

This disagreement sets the stage for a potential showdown between lawmakers and the president-elect on a sensitive issue: how much did the Russians weigh in on Trump's win.

McCain said the purpose of reviewing Russia's involvement in election hacking is not to call into question the outcome of the presidential election. However, such a review is vital and necessary to democracy and free and fair elections, he said. 

Many Republicans have been careful not to publicly criticise Trump's stances on Russian hacking. However, Thursday's hearing — televised by many news networks — also gives the intelligence community a more public platform to discuss their findings before a receptive body of lawmakers ahead of Trump's briefing.