By Roberta Rampton and Patricia Zengerle
WASHINGTON (Reuters) - President Donald Trump, striking a defiant tone on Thursday after days of political tumult, denied colluding with Russia during his 2016 campaign or asking former FBI Director James Comey to drop a probe into his former national security adviser.
"No. No. Next question," Trump told a news conference when asked if he "in any way, shape or form" ever urged Comey, whom he fired on May 9, to close down the investigation into Michael Flynn.
Comey's firing was one in a series of jarring developments that culminated on Wednesday in the Justice Department's appointment of a special counsel to probe possible ties between Russia and Trump's 2016 presidential campaign.
They included media reports that Trump discussed sensitive intelligence on Islamic State with Russia's foreign minister.
In a pair of morning tweets and at a later news conference, Trump described calls for his impeachment as "ridiculous" and said he had done nothing to warrant criminal charges.
"The entire thing has been a witch hunt and there is no collusion between certainly myself and my campaign - but I can always speak for myself - and the Russians. Zero," he said at a news conference with Colombian President Juan Manuel Santos.
Asked if he thought he had done anything to warrant criminal charges or impeachment as some on the left have demanded, the Republican president replied: "I think it's totally ridiculous."
Trump categorically denied he asked Comey to drop his probe of Flynn, whom he fired on Feb. 14 for misleading Vice President Mike Pence, about the extent of his conversations last year with Russia's ambassador.
According to multiple media reports, Comey wrote a memo detailing Trump's comments to him in February saying: "I hope you can let this go," referring to the Flynn probe.
In his earlier twitter posts, Trump decried the naming of Robert Mueller, a respected former FBI director, as a special counsel by Deputy Attorney General Rod Rosenstein, an official he himself appointed.
"With all of the illegal acts that took place in the Clinton campaign & Obama Administration, there was never a special counsel appointed!" Trump wrote on Thursday morning. He did not offer any evidence of such acts in his reference to Democratic former President Barack Obama and former presidential candidate Hillary Clinton.
"This is the single greatest witch hunt of a politician in American history!" added Trump.
Russia has denied U.S. intelligence agencies' conclusion that it interfered in the election campaign to try to tilt the vote in Trump's favor. Trump has long bristled at the notion that Russia played any role in his November election victory over Clinton.
Reuters reported on Thursday that Flynn and other Trump campaign advisers were in contact with Russian officials and others with Kremlin ties in at least 18 calls and emails during the last seven months of the presidential race.
Trump also said he was close to selecting a new FBI director to replace Comey, and that former senator and one-time Democratic vice presidential candidate Joe Lieberman was among the top candidates.
Trump's latest comments stood in contrast to his measured statement on Wednesday night after the announcement of Mueller's appointment, when he said "a thorough investigation will confirm what we already know - there was no collusion between my campaign and any foreign entity."
Democrats rejected Trump's characterization.
"This is a truth hunt," said Democratic Senator Amy Klobuchar.
Trump later told news anchors at the White House that Mueller's appointment was a "very, very negative thing," adding: "I believe it hurts our country terribly, because it shows we're a divided, mixed-up, not-unified country."
Rosenstein, the No. 2 Justice Department official, named Mueller amid mounting pressure in Congress for an independent investigation beyond existing FBI and congressional probes into the Russia issue.
Rosenstein briefed senators on Thursday, but made no public comments. Afterward, Republican Senator Lindsey Graham told reporters that "everything he said was that you need to treat this investigation as if it may be a criminal investigation."
Rosenstein told senators that he knew Comey would be fired before he wrote his letter, Democratic Senator Claire McCaskill said.
The White House initially said last week that a letter Rosenstein wrote accusing Comey of missteps as FBI director, including his handling of an election-year probe into Clinton's use of a private email server while she was secretary of state, was what prompted Trump to fire Comey. Trump later said he had already decided to dismiss him and was thinking of "this Russia thing."
A key issue Mueller may have to tackle is whether Trump has committed obstruction of justice, an offense that could be used in any effort in the Republican-led Congress to impeach him and remove him from office.
Asked about possible obstruction of justice, Republican House of Representatives Speaker Paul Ryan told reporters the special counsel would "follow the facts where ever they may lead" and that "it is premature to prejudge anything at this point."
(Reporting by Doina Chiacu and Patricia Zengerle; Additional reporting by Mark Hosenball, Susan Heavey, David Alexander and Amanda Becker; Writing by Will Dunham; Editing by Frances Kerry and Peter Cooney)