The US media reports cite a memo Comey allegedly wrote after he met with Trump in February. Commenting on the investigation into former National Security Advisor Michael Flynn, Trump reportedly told Comey that Flynn was a "good guy" who did nothing wrong.

"I hope you can let this go," Trump is said to have told Comey during the meeting in the White House.

Flynn resigned from his post just a day before the meeting on February 14, following reports that he had lied about the nature of his contacts with Russian diplomats.

The White House swiftly denied the Tuesday reports, first published by The New York Times, saying that the president "never asked Mr. Comey or anyone else to end any investigation, including any investigation involving General Flynn."

"This is not a truthful or accurate portrayal of the conversation between the president and Mr. Comey," the White House statement continued.

Comey 'taking notes' on Trump

The New York Times has reported that Comey made notes on the conversation with the president as part of a paper trail aimed to document perceived pressure from Trump. The then-FBI chief shared the existence of such memos with senior staff and his close aides at the agency.

While The New York Times reporters did not see the memo, one of Comey's associates read out parts of it to one of the journalists, the newspaper claims.

Trump fired Comey last week, and the White House said the move was motivated by Comey's handling of an investigation into former US Secretary of State Hillary Clinton's emails. Many observers have questioned this account, suggesting that Trump might instead have tried to hinder an ongoing FBI investigation into ties between Trump's team and Moscow.

After the dismissal, Trump appeared to make a veiled threat against Comey on Twitter, saying that the former official had "better hope there are no 'tapes' of our conversations" before he starts sharing information with the public.

FBI asked to provide memos

The Republican chairman of the House Oversight Committee, Jason Chaffetz, said he would seek to obtain Comey's note on meeting with Trump by way of subpoena - "if [the memo] exists," Chaffetz tweeted on Tuesday.

The memo raises concerns "about improper interference placed on an active investigation," Chaffetz told the AP news agency. He also published his letter to the FBI's acting director, Andrew McCabe, urging him to provide all of Comey's documents on meetings with the president within a week.

In a separate letter, the Democratic Party members on the House Oversight and Judiciary committees called for a joint investigation and a public hearing of Comey. The probe should clarify if Trump and his top aides were involved in a conspiracy to obstruct justice, the lawmakers said.

Uproar in the Senate

Senate Minority Leader Chuck Schumer said he was "shaken" by the reports of the president seeking to shut down the investigation into Flynn, and he warned his congressional colleagues that "history is watching."

"On a day when we thought things couldn’t get any worse, they have," the Democratic lawmaker from New York said in a brief speech on the Senate floor on Tuesday.

"Concerns about our national security, the rule of law, and the independence of our nation’s highest law enforcement agencies are mounting," he added. "The country is being tested in unprecedented ways."

Other Democrats have renewed calls that a special prosecutor should "immediately" be appointed to examine Trump's actions relating to the FBI.

Republican Senator Lindsey Graham, who has been a frequent Trump critic, said he would like to hear from Comey personally on the issue.

"Let's get to the bottom of what happened with the director. And the best way to get to the bottom of it is for him to testify," Senator Graham said. "I'm not going to take a memo, I want the guy to come in."

The Trump administration was facing enormous political pressure for firing Comey even before the latest reports, with the president notably contradicting his spokespeople on his handling of the dismissal.