US President Donald Trump has been slammed by fellow republicans and accused of “treason” after siding with his Russian counterpart Vladimir Putin over allegations of interference in the 2016 presidential election.

Speaking at a joint news conference after their summit in Helsinki on Tuesday morning (AEST), Mr Trump said the Russian collusion investigation has been a “disaster” for the US and has kept America and Russia “separated”.

Standing alongside Mr Putin, the US president said he saw no reason to believe Russia had hacked the 2016 US election to help him win and that Mr Putin “was extremely strong and powerful in his denial today”.

Mr Trump’s decision to side with Mr Putin against his own intelligence agencies came despite US special counsel Robert Mueller on Friday announcing the indictment of 12 Russian spies on charges of hacking Democratic Party computer networks during the election campaign.

Veteran Republican Senator John McCain called Mr Trump’s news conference “one of the most disgraceful performances by an American president in memory”.

Mr McCain said the summit with Mr Putin was “a tragic mistake”, saying Mr Trump proved not only unable, “but unwilling to stand up to Putin”.

He said the damage inflicted by Mr Trump’s “naiveté, egotism, false equivalence, and sympathy for autocrats is difficult to calculate”.

Former CIA director John Brennan went so far as to call for Mr Trump to be impeached over his stance at the news conference.

“Donald Trump’s press conference performance in Helsinki rises to & exceeds the threshold of ‘high crimes & misdemeanors,’ Mr Brennan posted on Twitter, referring to the standard for impeachment outlined in the US constitution.

“It was nothing short of treasonous. Not only were Trump’s comments imbecilic, he is wholly in the pocket of Putin. Republican Patriots: Where are you???”

Donald Trump’s press conference performance in Helsinki rises to & exceeds the threshold of “high crimes & misdemeanors.” It was nothing short of treasonous. Not only were Trump’s comments imbecilic, he is wholly in the pocket of Putin. Republican Patriots: Where are you???

— John O. Brennan (@JohnBrennan) July 16, 2018

US Senator Lindsey Graham, a senior Republican and member of the Senate Armed Services Committee, said Mr Trump’s news conference with Mr Putin would send Moscow a message of US “weakness”.

“Missed opportunity by President Trump to firmly hold Russia accountable for 2016 meddling and deliver a strong warning regarding future elections. This answer by President Trump will be seen by Russia as a sign of weakness and create far more problems than it solves,” he tweeted.

Mr Putin said he did want Donald Trump to win the 2016 presidential election, but took no action during the campaign to make it happen.

He said he favoured the celebrity businessman because of his policies.

“I had to repeat that the Russian state never interfered, and does not plan to interfere in internal American electoral process,” Mr Putin said.

Mr Trump said he and Mr Putin “spent a great deal of time” discussing allegations of Russian election meddling during hours-long summit Monday.

But Trump declined the opportunity to denounce Mr Putin for the interference efforts, which US intelligence agencies insist did occur.

Mr Trump said reiterated his stance that there was “no collusion” between his campaign and Russia.

“We ran a brilliant campaign and that’s why I’m president,” he said.

Later, in a tweet sent from Air Force One en route to the US, Mr Trump said he had confidence in US intelligence agencies, but said America could not “focus on the past”.

As I said today and many times before, “I have GREAT confidence in MY intelligence people.” However, I also recognize that in order to build a brighter future, we cannot exclusively focus on the past – as the world’s two largest nuclear powers, we must get along! #HELSINKI2018

— Donald J. Trump (@realDonaldTrump) July 16, 2018

At the news conference, Mr Trump said that he holds both the US and Russia responsible for years of strained relations, which he said he is now trying to improve.

Washington imposed sanctions on Moscow after Russia’s 2014 annexation of Crimea from Ukraine as well as for cyber attacks on the American electoral process.

Republican senator, Jeff Flake, on Twitter called Trump’s words “shameful”.

“I never thought I would see the day when our American President would stand on the stage with the Russian President and place blame on the United States for Russian aggression. This is shameful,” Mr Flake wrote.

I never thought I would see the day when our American President would stand on the stage with the Russian President and place blame on the United States for Russian aggression. This is shameful.

— Jeff Flake (@JeffFlake) July 16, 2018

Dan Coats, the US Director of National Intelligence, restated the American intelligence community’s assessment that Russia meddled in the 2016 election following the news conference.

“We have been clear in our assessments of Russian meddling in the 2016 election and their ongoing, pervasive efforts to undermine our democracy, and we will continue to provide unvarnished and objective intelligence in support of our national security,” he said.

-with agencies