President Donald Trump flatly denied Thursday that he ever urged ex-FBI Director James Comey to back off or close the investigation into former national security advisor Michael Flynn.

"No. No ... next question," Trump said when asked during a joint news conference with Colombian president Juan Manuel Santos if he had, in any way, tried to influence the probe. In a reported memo, Comey allegedly says Trump asked him to let the probe into Flynn go.

The president's comments come as he deals with backlash from firing Comey last week and subsequent reports that raised questions about whether he might have tried to interfere with an investigation. The FBI is investigating alleged Russian meddling in the 2016 election, including looking into any possible links between Trump campaign associates and the Kremlin.

Trump said there was "no collusion" but added that he can only speak for himself.

The president on Thursday muddled his explanation for firing Comey last week, saying he ousted him because he was "very unpopular with most people." He also highlighted the "very, very strong recommendation" he got from Deputy Attorney General Rod Rosenstein.

However, Trump told NBC News last week that he would have fired Comey "regardless" of what the Justice Department said. He added that he was thinking of the "Russia thing" when he did so.

Sen. Claire McCaskill, D-Mo., said after an all-senators briefing from Rosenstein that the deputy attorney general already knew Comey would be fired before he wrote a memo that the Trump administration initially used as justification for ousting the FBI chief.

Bipartisan lawmakers had criticized Comey for his handling of investigations related to Trump and his electoral opponent Hillary Clinton. But acting FBI Director Andrew McCabe told Congress last week that Comey "enjoyed broad support within the FBI and still does today."

After the Rosenstein briefing, Sen. Lindsey Graham, R-S.C., said the Russia probe "appears to me now to be considered a criminal investigation."

Watch: Javers on Trump's Comey comment