"This is a continuing pattern of disrespecting the men and women who serve in our intelligence community," Warner told reporters after he and Sen. Richard Burr met with Deputy Attorney General Rod Rosenstein.
Warner, the ranking democrat on the Senate Intelligence Committee, said he believes Trump would be "better served" if he supported the intelligence community, rather than questioning the integrity of its leaders.
Earlier Thursday, the president told NBC in an exclusive interview that he was going to fire Comey "regardless" of what the Justice Department recommended, contradicting what the White House said Wednesday.
"Look, he's a a showboat, he's a grand-stander, the FBI has been in turmoil. You know that, I know that. Everybody knows that. You take a look at the FBI a year ago, it was in virtual turmoil. Less than a year ago, it hasn't recovered from that," Trump said at the White House.
Sen. Richard Burr, chairman of the Senate Intel Committee, didn't directly respond to Trump's comments.
While the Republican senator acknowledged Trump's authority to determine who leads the FBI, Burr reiterated his previous statement on Comey's removal, saying he found him to be incredibly ethical. Burr said the "lion's share of FBI employees" respect Comey.
Sen. Richard Durbin, an intelligence committee member, chimed in via a tweet that Trump's interview comments showed the Russia investigation was a reason for firing Comey.
White House officials are scrambling to justify the timing of Comey's termination, insisting that he lost his job because of his conduct in the probe into Hillary Clinton's emails last year.
They said it had nothing to do with the FBI's probe into Russian meddling in the 2016 election, which Comey first revealed publicly in March and includes any possible links between the Trump campaign and Russia.
— CNBC's Jacob Pramuk contributed to this report.
Watch: More Americans think Comey firing 'inappropriate'