US President Donald Trump told Russian officials that firing FBI director James Comey eased "great pressure" on him, US media report.
Citing a document summarising the meeting, the New York Times said Mr Trump called Mr Comey a "real nut job".
Mr Comey had been running an inquiry into possible collusion between Russia and Donald Trump's election campaign.
The White House has not disputed the report.
"I just fired the head of the FBI. He was crazy, a real nut job," Mr Trump told the Russian officials, the Times reported.
"I faced great pressure because of Russia. That's taken off."
But warning that leaks were undermining national security, White House Press Secretary Sean Spicer said: "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."
Meanwhile, the Washington Post reported that a current White House official is a significant person of interest in the investigation into links between the Trump campaign and Russia.
The White House has denied there was collusion between the campaign and any "foreign entity".