Reaction on Capitol Hill was highly critical of Trump's performance in a joint news conference in Helsinki with Putin after the two leaders' first summit.

Republican Senator Lindsey Graham said it was a "missed opportunity by President Trump to firmly hold Russia accountable for 2016 meddling and deliver a strong warning regarding future elections".

Graham, a member of the Senate Armed Services Committee, said, "This answer by President Trump will be seen by Russia as a sign of weakness and create far more problems than it solves."

On Friday, a US special counsel announced indictments of 12 Russian spies on charges of hacking Democratic Party computer networks as part of the interference in the election campaign.

Republican Trump, standing alongside Putin in front of reporters on Monday, said he saw no reason to believe Russia had hacked the election to help him win and that Putin "was extremely strong and powerful in his denial today".

House of Representatives Speaker Paul Ryan said in a statement that Russia undoubtedly interfered in the 2016 election.

"The president must appreciate that Russia is not our ally. There is no moral equivalence between the United States and Russia, which remains hostile to our most basic values and ideals," Ryan said.

Another Republican, Senator John McCain, described Trump's summit with Putin as "a tragic mistake".

Senate Majority Leader Mitch McConnell's office said he had no immediate comment on Trump's remarks in Helsinki.

Top House of Representatives Democrat Nancy Pelosi wrote on Twitter, "Every single day, I find myself asking: what do the Russians have on (at)realDonaldTrump personally, financially, & politically? The answer to that question is that only thing that explains his behaviour & his refusal to stand up to Putin."

Trump said he holds both the US and Russia responsible for years of strained relations.

The US president had touted the summit as an opportunity to reduce tensions, inflamed by Russia's annexation of the Crimea peninsula from Ukraine in 2014, its military backing of Syrian President Bashar al-Assad, which turned the tide of the Syrian civil war in 2015, and accusations of Russian meddling in the 2016 US election.

Senate Democratic leader Charles Schumer said Trump's approach in Helsinki was unprecedented.

"For the president of the United States to side with President Putin against American law enforcement, American defence officials, and American intelligence agencies is thoughtless, dangerous, and weak," Schumer said.

Republican Senator Jeff Flake 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," Flake said on Twitter.

Representative Adam Schiff, senior Democrat on the House of Representatives intelligence panel, predicted Putin will take Trump's remarks "as a green light to interfere" in 2018 congressional elections. Schiff called Trump "cowardly".

At the Helsinki summit, Putin gave Trump a soccer ball from the World Cup hosted by Russia during the past month and as an acknowledgement the US will co-host the 2026 tournament.

Senator Graham afterward tweeted, "Finally, if it were me, I'd check the soccer ball for listening devices and never allow it in the White House."

Australian Associated Press