The President allegedly made the remarks amid the Justice Department’s snowballing probe into a possible collusion between his election campaign and Russian government officials.
The White House has not addressed the recent remarks but a spokesman said that Mr Comey had “created unnecessary pressure” on Russian relations.
Mr Trump has repeatedly dismissed the allegations about ties with Russia as “fake news”.
In a statement, White House press secretary Sean Spicer said: “The President has always emphasised the importance of making deals with Russia as it relates to Syria, Ukraine, defeating ISIS and other key issues for the benefit and safety of the American people.
“By grandstanding and politicising the investigation into Russia’s actions, James Comey created unnecessary pressure on our ability to engage and negotiate with Russia. The investigation would have always continued, and obviously, the termination of Comey would not have ended it.
“Once again, the real story is that our national security has been undermined by the leaking of private and highly classified conversations.”
US media reports also allege Mr Trump told the Russians he had “taken off” pressure over the investigation by firing Mr Comey.
He allegedly told the Russians: “I just fired the head of the FBI. He was crazy, a real nut job.
“I faced great pressure because of Russia. That’s taken off.”
Deputy Attorney General Rod Rosenstein sent shockwaves through Washington this week by appointing former FBI director Robert Mueller as special prosecutor to spearhead the probe.
Mr Mueller’s appointment came in the wake of Mr Comey’s firing, who originally oversaw the investigation.
On Thursday, leading Republican Senator Lindsey Graham said he believed the Trump-Russia investigation was now “considered a criminal investigation”.