House Speaker Paul Ryan has said there is "no question" that Russia interfered in the 2016 presidential election and continues to try undermine democracy in the United States and around the world.
The Wisconsin Republican said the American intelligence community and the House Intelligence Committee agree that Russia interfered in the election.
He added: "The president must appreciate that Russia is not our ally."
Mr Ryan said that Russia "remains hostile to our most basic values and ideals".
"The United States must be focused on holding Russia accountable and putting an end to its vile attacks on democracy," he added.
Mr Ryan's comments came after President Donald Trump said at a summit with Russian President Vladimir Putin that he sees no reason why Russia would interfere in the US election.
The Kremlin has denied any state action in the election.
Meanwhile, Senator John McCain called President Trump's press conference "one of the most disgraceful performances by an American president in memory".
The Arizona Republican said the summit was "a tragic mistake".
The senator said President Trump proved not only unable, "but unwilling to stand up to Putin".
Sen. McCain, who has been away from the Senate as he battles brain cancer, said the damage inflicted by President Trump's "naivete, egotism, false equivalence, and sympathy for autocrats is difficult to calculate".
And the director of National Intelligence later restated the US intelligence community's assessment that Russia meddled in the 2016 election.
Dan Coats said the intelligence community's role is to provide "the best information and fact-based assessments possible for the president and policymakers".
He added: "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."