WASHINGTON (REUTERS) - A small but growing number of US President Donald Trump's fellow Republicans called on Wednesday (May 17) for an independent probe of possible collusion between his 2016 campaign and Russia, and one even mentioned impeachment, spurred by a memo from the fired FBI chief that Trump tried to impede the agency's investigation.

The tumult in Washington deepened over allegations Trump had sought to end the FBI's investigation into ties between Trump's first national security adviser, Michael Flynn, and Russia.

US stocks were on track for their worst day in almost two months and the dollar dropped as investors fled risky assets amid uncertainty about the president's ability to deliver on tax and regulatory changes.

Prices of bonds, regarded as a safe haven in times of market turbulence, rallied.

House of Representatives Speaker Paul Ryan insisted the Republican legislative agenda was not becoming paralysed.

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, a source who has seen a memo written by Comey said on Tuesday.

Trump removed Comey in the middle of an FBI investigation into alleged Russian meddling in the 2016 US presidential election, and possible ties between Trump's campaign and Russia. In a separate but related probe, the FBI is looking into Flynn's ties with Russia.

Democratic lawmakers have demanded that the Justice Department name a special prosecutor to investigate the Russia matter.

Asked by reporters whether he thought the allegations against Trump were grounds for impeachment, Republican Representative Justin Amash said, "If the allegations are true, yes. But everybody in this country gets a fair trial, including the president or anyone else."

Amash is a member of the hard-line House Freedom Caucus who has already has called for an independent commission to investigate.

Asked if he trusted Comey or Trump, Amash replied:"I think it's pretty clear I have confidence in Director Comey."

It would be significant if more Republicans began to talk about impeaching the president, as the party holds a majority in both chambers of Congress. A simple majority is required in the House to impeach a president, which would lead to a trial by the Senate and possible removal from office.

Republican Senator Lisa Murkowski said on Wednesday it may be time for an independent commission or special prosecutor, expressing concern about "the continuous and often conflicting reports about President Trump, the FBI and Russia."

"The American people deserve to know the truth," she said, adding it is imperative to restore the public's trust.

Republican Adam Kinzinger, a member of the House of Representatives Foreign Affairs Committee, was among Republicans calling for an independent probe.

He told CNN on Wednesday, "I think we are in the position now where it's time for an independent commission or a special prosecutor or whatever."

Republican Representative Steve Knight on Tuesday said he joined "many of my colleagues in supporting the assignment of a special prosecutor to take over the ongoing FBI investigation."

As the controversy swirled in Washington, Trump had defiant words during a speech to US Coast Guard Academy graduates in Connecticut, complaining about media coverage and saying no politician in history "has been treated worse or more unfairly."

Trump said that "adversity makes you stronger," adding that things are not always fair, but "you have to put your head down and fight, fight, fight. Never ever, ever give up. Things will work out just fine."

The Comey memo caused alarm on Capitol Hill and raised questions about whether Trump attempted to interfere with a federal investigation, something that might constitute obstruction of justice and could potentially be invoked in an attempt to impeach Trump.

The White House denied the report, saying it was "not a truthful or accurate portrayal of the conversation between the president and Mr. Comey."

It followed a week of chaos at the White House after Trump fired Comey on May 9.

Criticism of the president intensified after it emerged on Monday (May 15) that the president 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.

Russian President Vladimir Putin said on Wednesday that Trump had not passed on any secrets to Lavrov. Putin made light of the matter at a news conference in Russia, saying Moscow was ready to hand a transcript of the meeting to US lawmakers if that would help reassure them.

RYAN'S CONFIDENCE IN TRUMP

Most Republicans have said the current Federal Bureau of Investigation probe and investigations in the Republican-led Congress into the Russia matter are sufficient.

Ryan stuck to that line and told reporters on Wednesday he still has confidence in Trump. "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."

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.

Republican Senator John McCain, who has called for the creation of a special bipartisan congressional panel to investigate the Russia matter, compared the controversies enveloping Trump to the Watergate scandal that forced Republican President Richard Nixon to resign in 1974.

"I think it's reaching the point where it's of Watergate size and scale, and a couple of other scandals we've seen,"McCain said at a Washington event on Tuesday night.

In a letter to acting FBI Director Andrew McCabe on Tuesday, the Republican chairman of a House 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.