The White House has condemned a New York Times report that the FBI opened an inquiry into whether President Trump was secretly working for Russia.
Law enforcement officials became concerned by Mr Trump's behaviour in May 2017, when he sacked FBI director James Comey, the paper says.
The investigation reportedly examined whether Mr Trump was a national security threat.
Mr Trump said there was no reason and no proof for setting up such a probe.
— Donald J. Trump (@realDonaldTrump) 12 janvier 2019
"This is absurd," White House press secretary Sarah Huckabee Sanders said.
"James Comey was fired because he's a disgraced partisan hack, and his deputy Andrew McCabe, who was in charge at the time, is a known liar fired by the FBI," she said in a statement.
"Unlike President Obama, who let Russia and other foreign adversaries push America around, President Trump has actually been tough on Russia."
In 2016, US intelligence agencies concluded that Russia had launched cyber-attacks and planted fake news stories on social media in a bid to boost Donald Trump and damage his rival for the presidency, Hillary Clinton.