President Donald Trump found himself isolated and under pressure to reverse course Tuesday after publicly challenging the US intelligence conclusion that Russia meddled in the 2016 election during his face-to-face with Vladimir Putin.

At his inaugural summit with the Russian president in Finland, Trump appeared to accept at face value the strongman's denial that Moscow interfered in a bid to undermine his Democratic rival Hillary Clinton -- a stance that triggered bipartisan outrage at home.

Back in Washington, Trump sounded a defensive note, insisting his meeting with Putin had been "even better" than his one last week with traditional allies NATO -- a testy gathering seen as having badly strained trans-Atlantic ties.

But the US president has found precious little support for his failure to confront Putin, and faced calls even from allies to change tack.

"He has to reverse course immediately and he's gotta get out there as soon as possible before the concrete starts to set on this," former White House communications director Anthony Scaramucci said on CNN.

"Loyalty right now requires you to tell the truth and sit with him and explain to him the optics of the situation, why the optics are bad, the strategy in terms of trying to get along with Vladimir Putin and deploying a strategy of going against the intelligence agency is very bad," Scaramucci said.

Former House speaker Gingrich and longtime Trump ally Newt Gingrich put it yet more bluntly.

"President Trump must clarify his statements in Helsinki on our intelligence system and Putin," he tweeted as Trump headed home from Helsinki. "It is the most serious mistake of his presidency and must be corrected -- immediately.

Trump's performance at the summit has even come under fire from the hosts at Fox News, usually a reliable defender of the president.

"No negotiation is worth throwing your own people and country under the bus," Fox anchor and Fox & Friends co-host Abby Huntsman wrote on Twitter.

Trump and Putin met for two hours in Helsinki on Monday with only their interpreters present, then held a joint press conference.

Standing alongside the Kremlin boss, Trump acknowledged that his intelligence chiefs believe Russia hacked and leaked Democrats' emails containing politically damaging information about his rival Hillary Clinton in 2016.

'Undermine democracy'

But, insisting he had won the race fair and square, the Republican said: "I have President Putin, he just said it is not Russia. I will say this: I don't see any reason why it would be."

The US leader insisted that his counterpart had delivered a "powerful" denial of any Russian manipulation, and that the investigation by special counsel Robert Mueller, into Russian meddling and possible collusion with the Trump campaign, was proving a "disaster" for the United States.

Mueller's investigation has increasingly put pressure on the White House, and Trump -- who regards it as an attack on the legitimacy of his presidency -- has dubbed it a "witch hunt."

But the investigation continues to progress, resulting in the indictment of 12 Russian military intelligence agents on Friday -- timing that was embarrassing in light of the upcoming summit.

While Trump has faced intense criticism over Helsinki, he is not entirely without defenders.

Republican Senator Rand Paul has given a series of interviews supporting Trump's stance towards Putin, and berating his critics as biased against the president.

"I think the president did a good thing by meeting with Putin and I think it's a mistake for people to try to turn this into a partisan escapade," Paul said on CBS.

"This is truly the Trump derangement syndrome that motivates all of this," the Kentucky Republican told CNN.

Paul's efforts drew praise from Trump, who tweeted: "Thank you @RandPaul, you really get it!"

But the bipartisan consensus has been broadly hostile to Trump's stance -- as the top Republican on Congress, House Speaker Paul Ryan made clear once more at a press conference Tuesday on Capitol Hill.

"We stand by our NATO allies and all those countries who are facing Russia aggression," Ryan said. "Vladimir Putin does not share our interests, Vladimir Putin does not share our values."

"Russia did meddle with our elections. Not only did Russia meddle with our elections, they are doing it around the world," he said.

"Russia is trying to undermine democracy itself."