WASHINGTON - US President Donald Trump, striking a defiant tone on Thursday after days of political tumult, denied asking former FBI Director James Comey to drop a probe into his former national security adviser and decried a "witch hunt" against him.Trump's terse denial followed reports by Reuters and other media about a memo written by Comey alleging that Trump made the request to close down the investigation into Michael Flynn and Russia in February. Trump fired Comey on May 9."No. No. Next question," Trump told a news conference in the White House, when asked if he "in any way, shape or form" ever urged Comey to end the probe.Comey's dismissal last week set off 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 the Islamic State militant group with Russia's foreign minister.In a pair of morning Twitter posts and at a later news conference, the Republican president described calls by some on the left 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 told the news conference, standing alongside Colombian President Juan Manuel Santos.In his earlier Twitter posts, Trump criticized the naming of former FBI Director Robert Mueller 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 former Democratic President Barack Obama and former presidential candidate Hillary Clinton."This is the single greatest witch hunt of a politician in American history!" Trump tweeted.Democrats rejected Trump's characterization."This is a truth hunt," said Democratic Senator Amy Klobuchar.Russia has denied US 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.Trump fired Flynn on Feb. 14 for misleading Vice President Mike Pence about the extent of his conversations last year with Russia's ambassador.