Democrats and several prominent Republicans have raised concerns over the timing of the move
Donald Trump has defended his controversial sacking of James Comey - claiming the FBI director "was not doing a good job".
The US president met Russian Foreign Minister Sergei Lavrov in the White House on Wednesday, less than 24 hours after removing the man in charge of investigating alleged links between Trump presidential campaign officials and the Kremlin.
The timing of the decision sparked outrage in Washington, with Democrats in the Senate "virtually unanimous" that a special prosecutor must now be appointed to take over the Russia investigation, according to one lawmaker.
Trump administration officials have denied any link between the sacking and the FBI's inquiry - claiming it had more to do with his handling of the investigation into Hillary Clinton's e-mail use.
As the president delivered his assessment of Mr Comey's work, reports emerged that America's top law enforcement official had asked for more money and manpower for his Russia investigation in the days before he was fired.
The New York Times said Mr Comey approached Deputy Attorney-General Rod Rosenstein for extra funds last week and later briefed lawmakers on the request - a report denied by the Justice Department.
US Vice President Mike Pence backed Mr Trump's decision to fire the FBI boss, saying the "President made the right decision at the right time".
Republican Senator Richard Burr, chair of the Senate Intelligence Committee, said there was no need for a special prosecutor to continue the Russia investigation.
He said his committee had the jurisdiction and responsibility to proceed and "we are going to do that."
But he admitted Mr Comey's departure could slow down the committee's work and added: "The timing of this and the reasoning for it doesn't make sense to me."
The White House said the president urged Mr Lavrov to "rein in" the Assad regime in Syria during their meeting.
Mr Trump said: "We had a very, very good meeting with Mr. Lavrov. We want to see the killing, the horrible killing, stopped in Syria as soon as possible and everyone is working toward that end."
Mr Lavrov said the meeting with Mr Trump had been "free of the ideology" that hampered US-Russia relations under during Barack Obama's time in office.
He also dismissed claims of Russian meddling in US domestic politics as "fabrications".
The bureau editor for BBC North America Paul Danahar has tweeted, claiming the meeting was closed to the press and the only visual account given was handout photos from the Russian government.
Taken from today's White House Pool report. It's quite something. pic.twitter.com/25FjkOypEi— Paul Danahar (@pdanahar) May 10, 2017
Earlier, the Russian foreign minister had sarcastically acknowledged Mr Comey's dismissal during a meeting with US Secretary of State Rex Tillerson.
Asked by a reporter if the firing would cast a shadow over his talks, Lavrov replied in a sarcastic tone: "Was he fired? You're kidding. You're kidding."
Earlier on Wednesday, Mr Trump had followed up the shock sacking with a series of tweets attacking Mr Comey.
The president moved to justify his decision by insisting that Mr Comey had "lost the confidence of almost everyone in Washington".
Comey lost the confidence of almost everyone in Washington, Republican and Democrat alike. When things calm down, they will be thanking me!— Donald J. Trump (@realDonaldTrump) May 10, 2017
"James Comey will be replaced by someone who will do a far better job, bringing back the spirit and prestige of the FBI," he wrote.
As well as retweeting an article which set out "10 major FBI scandals on Comey's watch", Mr Trump accused the Democrats of hypocrisy.
"The Democrats have said some of the worst things about James Comey, including the fact that he should be fired, but now they play so sad!" he tweeted.