Trump claims Comey told him he is not under investigation as reports emerge that Comey sought to expand Russia probe
President Donald Trump asserted in his extraordinary letter firing James Comey that the former FBI director told him three times he was not under investigation.
If true, that claim would be a startling breach of protocol.
Former Justice Department officials and federal prosecutors said they doubted Mr Comey would have spoken to President Trump in those terms - or even offered any assurances at all.
Mr Comey has publicly confirmed the FBI is investigating possible contacts between the Trump presidential campaign and Russia.
President Trump offered no details about exactly what he was told, or when.
The White House declined to provide back-up or explanation for President Trump's assertion in the letter.
Meanwhile, a congressional source has said that Mr Comey sought to expand his agency's probe into possible collusion between Trump's presidential campaign and Russia to sway the 2016 US election.
With the Republican president facing a storm of criticism from many Democratic lawmakers and some in his own party, the Trump administration accused Comey of "atrocities" on the job and denied his firing was related to the FBI's Russia investigation.
Trump, who met Russia's foreign minister at the White House on Wednesday, lashed out at critics, calling Democrats "phony hypocrites," and defended his decision to abruptly oust Comey on Tuesday from the law enforcement post he held since 2013.
Democrats intensified accusations that Comey's removal was intended to undermine the Federal Bureau of Investigation probe and demanded an independent investigation into the alleged Russian meddling, with some calling the firing an attempt to cover up wrongdoing related to Russia. Some of Trump's fellow Republicans called the action troubling.
The firing stunned Washington and plunged Trump deeper into a controversy over his campaign's alleged ties with Russia that has dogged the early days of his presidency, while also threatening to hinder his policy goals.
"He wasn't doing a good job, very simply," the Republican president said of Comey during a meeting with former Secretary of State Henry Kissinger in the White House Oval Office.
A congressional source with knowledge of the matter said Comey told lawmakers within the past few days he had asked the Justice Department for additional resources - mainly more staffing - for the Russia probe. Comey informed lawmakers of that request after the Senate intelligence committee, conducting its own investigation, had asked the FBI to speed up its Russia inquiry, the source said.
Democrat Dianne Feinstein, the Senate Judiciary Committee's top Democrat, told reporters she understood Comey was seeking more resources for the FBI investigation.
"We know that there are subpoenas being requested in the Eastern District of Virginia, and that this investigation has been going on," Feinstein told reporters.
Responding to media reports that Comey had asked Deputy Attorney General Rod Rosenstein last week for a significant boost in resources for the agency's probe, Justice Department spokesman Ian Prior said in an email: "Totally false."
US intelligence agencies concluded in a January report that Russian President Vladimir Putin had ordered an effort to disrupt the 2016 election that included hacking into Democratic Party emails and leaking them, with the aim of helping Trump.
Russia has denied any such meddling. The Trump administration denies allegations of collusion with Russia.
The administration said on Tuesday that Comey's firing stemmed from his handling of an election-year FBI probe into Democratic presidential nominee Hillary Clinton's use of a private email server during her time as secretary of state.
White House officials said Trump's anger at Comey had been building for months but a turning point came when the FBI chief refused to preview for top Trump aides his planned testimony to a May 3 Senate hearing on the Clinton email issue, an act Trump and his aides took it as an act of insubordination.
Trump had been considering letting Comey go "since the day he was elected" in November, White House spokeswoman Sarah Huckabee Sanders said. She referred to what she called Comey's "atrocities in circumventing the chain of command" at the Justice Department.