President Donald Trump is accusing Democrats of fabricating allegations of election interference against Russia and creating more "fake news."
His tweets came just hours before a congressional hearing on Russia on Monday (Tuesday AEDT).
The House Intelligence Committee will hear from FBI Director James Comey on whether US officials believe Russia tried to bolster Trump's chances in the election and if there were any connections between Moscow and Trump's campaign aides.
Trump tweeted on Monday: "The Democrats made up and pushed the Russian story as an excuse for running a terrible campaign. Big advantage in Electoral College & lost!"
A separate tweet referenced James Clapper, the director of national intelligence under President Barack Obama, who has said that Trump's allegation that Obama wiretapped Trump Tower is false.
"James Clapper and others stated there is no evidence Potus colluded with Russia. This story is FAKE NEWS and everyone knows it."
Representatives Devin Nunes, chairman of the House of Representatives Permanent Select Committee on Intelligence, and Adam Schiff, the panel's top Democrat, have called Comey and NSA Director Admiral Mike Rogers to testify as part of their committee's probe into allegations that Russia meddled in US elections.
Other congressional committees also are investigating the matter, mostly behind closed doors. But amid a furore over whether Moscow tried to influence the 2016 presidential race on Trump's behalf, politicians said they would make public as much of their probes as possible.
Russia denies attempting to influence the election.
Comey and Rogers are not expected to reveal much in public about the probes, which include information that is classified Top Secret and also separated into different compartments, each of which requires a separate clearance.
But the hearing could become heated as Republicans balance support for their party's leaders and Democrats vent frustration over Republican congressional leaders' refusal to appoint a special prosecutor or select committee to investigate.
Trump fired his first national security adviser, Michael Flynn, last month after he failed to disclose contacts with Russia's ambassador before Trump took office on January 20.
Last week, new information surfaced about more than $US65,000 ($A84,182) that Flynn was paid in 2015 by companies with links to Russia.
Attorney General Jeff Sessions, a former senator, recused himself from investigating the matter after it was revealed that he did not answer accurately when he was asked during his confirmation hearing about his contacts with Russian officials during the election. He failed to disclose that, as senator, he had met with Russia's ambassador to the United States, Sergey Kislyak.
