By Susan Heavey and Doina Chiacu
WASHINGTON (Reuters) - A small but growing number of Republican U.S. lawmakers called on Wednesday for an independent probe of possible collusion between President Donald Trump's 2016 campaign and Russia, spurred by a memo from the fired FBI chief that Trump had sought to impede the agency's investigation.
The tumult in Washington deepened on Tuesday over allegations Trump had sought to end the FBI's investigation into ties between Trump's first national security adviser, Michael Flynn, and Russia.
James Comey, whose firing as FBI director last week triggered a political firestorm, wrote a memo detailing Trump's comments to him in February saying "I hope you can let this go," referring to the Flynn probe, according to a source who has seen a memo written by Comey.
The Comey memo caused alarm on Capitol Hill and raised questions about whether Trump attempted to interfere with a federal investigation. The White House denied the report, saying it was "not a truthful or accurate portrayal of the conversation between the president and Mr. Comey."
U.S. stocks opened sharply lower on Wednesday after the reports of the memo.
Democratic lawmakers have demanded that the Justice Department name a special prosecutor to investigate potential ties between Trump's campaign and Russia.
But most Republicans have said the current Federal Bureau of Investigation probe and investigations in the Republican-led Congress are sufficient.
House of Representatives Speaker Paul Ryan stuck to that line and told reporters on Wednesday he still has confidence in Trump. Ryan also insisted the Trump controversies were not paralyzing Republican legislative efforts in Congress.
"We need the facts," Ryan said. "It is obvious there are some people out there who want to harm the president. But we have an obligation to carry out our oversight regardless of which party is in the White House."
"I'm sure we're going to go on to hear from Mr. Comey about why, if this happened as he allegedly describes, why didn't he take action at the time? So there are a lot of unanswered questions."
A few Republicans have began to call for an independent probe.
"If in fact what was in the memo is true, it's very concerning, and we need to get to the bottom of that," Republican Adam Kinzinger, a member of the House of Representative Foreign Affairs Committee, said on CNN. "I think we are in the position now where it's time for an independent commission or a special prosecutor or whatever."
The latest developments have overshadowed the policy goals of Trump's Republican Party in Congress, including major healthcare legislation and tax cuts.
Representative Adam Schiff, the top Democrat on the House Intelligence Committee, said pressure was mounting on Ryan to allow a vote on legislation creating an independent commission to take up the investigation.
'FOLLOW THE EVIDENCE'
Reports about the Comey memo followed a week of chaos at the White House after Trump fired Comey.
Criticism of the president intensified after it emerged on Monday that he had discussed sensitive national security information about Islamic State with Russian Foreign Minister Sergei Lavrov and Russia's ambassador in Washington at a White House meeting last week.
Some legal analysts have said Trump's possible pressure on Comey to end the Flynn probe and his decision to fire the FBI chief in the midst of the agency's investigation of the Russia matter could amount to obstruction of justice, a charge that could be invoked in any attempt to impeach Trump and remove him from office.
Schiff said impeachment is "not something to be lightly entertained. I think we follow the evidence, we obtain the evidence, we hear the testimony and then we decide just what does this show about the president's conduct."
Comey wrote the memo after he met in the Oval Office with Trump, the day after the president fired Flynn on Feb. 14 for misleading Vice President Mike Pence about the extent of his conversations last year with Russia's ambassador, Sergei Kislyak.
In a letter to acting FBI Director Andrew McCabe on Tuesday, the Republican chairman of a House of Representatives oversight committee, Jason Chaffetz, set a May 24 deadline for the FBI to produce all relevant material relating to any communications between Comey and Trump. Ryan backed Chaffetz's request.
Russian President Vladimir Putin said on Wednesday Trump had not passed on any secrets to Lavrov during their meeting. Making light of the matter at a news conference in the Russian city of Sochi, Putin said Russia was ready to hand a transcript of the meeting to U.S. lawmakers if that would help reassure them.
A Kremlin aide, Yuri Ushakov, later told reporters Moscow had a written record of the conversation, not an audio recording.
(Additional reporting by Steve Holland, Amanda Becker, Doina Chiacu, Tim Ahmann, Patricia Zengerle and Julia Edwards Ainsley in Washington, and Jan Wolfe in New York; Writing by Will Dunham; Editing by Frances Kerry)