Trump denies asking Comey to drop probe, decries 'witch hunt'

US President Donald Trump denied asking former FBI director James Comey to drop a probe into his former national security adviser and decried a "witch hunt" against him.

During a news conference at the White House Thursday, Trump rejected any suggestions that he tried to interfere with the investigation. He assailed the FBI probe, saying that it unfairly targets him.

"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.

Trump fired Comey on 9 May. 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. Former FBI Director Robert Mueller was appointed to oversee the investigation.

Other developments include 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.

Trump fired Flynn on 14 February for misleading Vice President Mike Pence about the extent of his conversations last year with Russia's ambassador.

Russia has denied US intelligence agencies' conclusion that it interfered in the election campaign to try to tilt the vote in Trump's favour. Trump has long bristled at the notion that Russia played any role in his November election victory over Clinton.