President Donald Trump personally appealed to FBI director James Comey to abandon the bureau's investigation into National Security Adviser Michael Flynn, according to reports.
The White House issued a furious denial near the end of a tumultuous day. Earlier officials staunchly defended Mr Trump's disclosure of classified information to senior Russian officials.
The White House played down the importance and secrecy of the information, supplied by Israel under an intelligence-sharing agreement, and Mr Trump himself said he had "an absolute right" as president to share "facts pertaining to terrorism" and airline safety with Russia.
As for Mr Comey, who Mr Trump fired last week, the FBI director wrote in a memo after a February meeting at the White House that the new president had asked him to shut down the FBI's investigation of Mr Flynn and his Russian contacts, said a person who had read the memo.
The Flynn investigation was part of a broader probe into Russian interference in last year's presidential election.
The existence of the memo was first reported on Tuesday by The New York Times.
The White House vigorously denied the report. "While the president has repeatedly expressed his view that General Flynn is a decent man who served and protected our country, the president has never asked Mr Comey or anyone else to end any investigation, including any investigation involving General Flynn," a White House statement said.
Mr Trump fired Mr Flynn on February 13.
When Mr Trump fired Comey, he said he did so based on Mr Comey's very public handling of the Hillary Clinton email probe and how it affected his leadership of the FBI.
Representative Jason Chaffetz, Republican chairman of the House intelligence committee, said he was ready to subpoena the memo, which was an apparent effort to create a paper trail of Mr Comey's contacts with the White House.
Other politicians also demanded strong action.
Democratic Senator Dianne Feinstein of California said Mr Comey needs to come to Capitol Hill and testify.
Mark Warner of Virginia, top Democrat on the Senate intelligence committee, said he would ask Mr Comey for additional material as part of the panel's investigation. "Memos, transcripts, tapes - the list keeps getting longer," he said.
John McCain, chairman of the Senate Armed Services Committee, said the Trump-Russia reports were "deeply disturbing" and said they could impede allies' willingness to share intelligence with the US
Mitch McConnell, the Republican leader of the Senate, said he was not losing confidence in Mr Trump as president but added, "It would be helpful to have less drama emanating from the White House."
According to the Times, Mr Comey wrote in the February memo that Mr Trump told him Mr Flynn had done nothing wrong. But Mr Comey said he did not say anything to Mr Trump about limiting the investigation, replying simply, "I agree he is a good guy."
The newspaper said Mr Comey was in the Oval Office that day with other national security officials for a terrorism threat briefing. When that ended, Mr Trump asked everyone to leave except Mr Comey, and he eventually turned the conversation to Mr Flynn.
The administration spent the first half of Tuesday defending Mr Trump's disclosure of classified information to senior Russian officials. National Security Adviser HR McMaster said the president's comments were "wholly appropriate". He used that phrase nine times in his briefing to reporters.
The highly classified information about an Islamic State plot was collected by Israel, a crucial source of intelligence and close partner in the fight against some of the America's fiercest threats in the Middle East.
Mr Trump's disclosure of the information threatened to fray that partnership and piled pressure on the White House to explain the apparently on-the-spot decision to reveal the information to Russian diplomats in the Oval Office.
In a statement, Israel's ambassador to the US, Ron Dermer, said the partnership between the U.S and Israel was solid.