The man privy to America's darkest intelligence secrets for the past six years has predicted Russian links to the Trump administration will dwarf the 1970s Watergate scandal which sunk the Nixon presidency.

Key points: Former US intelligence chief James Clapper says Watergate scandal nothing compared to Trump's Russia links

Former US intelligence chief James Clapper says Watergate scandal nothing compared to Trump's Russia links Says Russian meddling in US election constitutes an 'assault' on the United States

Says Russian meddling in US election constitutes an 'assault' on the United States Clapper slams Trump administration's hostility towards US government institutions

James Clapper was until January the US director of national intelligence — the Obama administration's eyes and ears on America's enormous surveillance community, including the CIA, NSA and elements of the FBI.

In that role he watched Russian meddling in the months leading up to the presidential election last November, and has concluded it made a concerted and determined attempt to derail Mr Trump's opponent, Hillary Clinton.

"I lived through Watergate. I was on active duty then in the Air Force, I was a young officer. It was a scary time," Mr Clapper told the National Press Club in Canberra.

"I have to say though, I think if you compare the two that Watergate pales, really, in my view, compared to what we're confronting now."

President Richard Nixon was forced to resign in 1974 after it was revealed Republican operatives had broken into the Watergate Hotel to bug rooms used by their Democratic opponents.

Russian meddling an 'assault' on the US

Mr Clapper said Russia's interference in the 2016 presidential election was an "assault" on the US, and the President's pro-Russia stance was "inexplicable".

"Social media trolls planted false information, orchestrated fake news — which many other news outlets picked up and amplified either wittingly or unwittingly — and a very sophisticated campaign by the regime-funded propaganda arm … against Hillary Clinton and for Donald Trump" he said.

"Their first objective was to sow doubt, discontent and discord about our political system. They achieved, I'm sure, beyond their wildest expectations. Given their success, they have only been emboldened to be even more aggressive in the future."

Bob Mueller is currently assembling a team to investigate possible collusion between Russia and Trump's election campaign. ( Reuters )

Former FBI chief Robert Mueller is currently assembling a team to investigate possible collusion between Russia and Mr Trump's campaign.

Mr Clapper said he was bewildered the President was not more hostile to the Russian regime.

"I've had a real hard time reconciling the threat the Russians pose to the United States and, by extension, Western democracies in general, with inexplicably so solicitous stance the Trump administration, or others in it, has taken with respect to Russia," he said.

Mr Clapper's speech came just days ahead of a highly anticipated Senate committee hearing with sacked FBI director James Comey.

The US media has reported Mr Trump tried to pressure Mr Comey to drop an investigation into Moscow's links with the President's then national security adviser.

Mr Clapper said he would be "very interested" to hear Mr Comey's evidence — who he said had been "inexcusably" sacked by Mr Trump.

"As I have often said, it is absolutely crucial for the United States, and for that matter for the world, for this presidency, for the Republicans, for the Democrats and for our nation at large, that we get to the bottom of this," he said.

"Is there a smoking gun with all the smoke? I don't know the answer to that. I think it's vital, though, we find that out."

US institutions 'under assault'

Mr Clapper also launched a furious attack on Mr Trump's performance in office, saying his hostility towards US government institutions did enormous damage to the country.

Former top US spy James Clapper says he will be interested to hear sacked FBI director James Comey's evidence. ( Reuters: Kevin Lamarque )

"I'm ever the optimist so I do have great faith and confidence — even though they are under assault — in our basic institutions," he said.

"That is not unlimited, though, so the question is: how long can these assaults go on and the institutions not be irrevocably damaged?"

He also gave a withering account of Mr Trump's behaviour since he came to office. He accused the President and his administration of disparaging the intelligence community's assessment of serious Russian interference in the election.

"This was prompted, I found … by his and his team's extreme paranoia about and resentment of any doubt cast on the legitimacy of his election, which of course our assessment did."

He said when Mr Trump made his now famous visit to CIA headquarters, he hoped the President would acknowledge their contribution to protecting the United States.

"Instead, he chose to use that as a prop for railing about the size of the inauguration crowd on the Mall and his battle with the fake news media."