Witch Hunt, says Trump confirming he is being investigated for firing Comey

US President Donald Trump appeared to confirm that he was being probed for firing the FBI chief and described it as a "witch hunt", as he asserted that there was no proof of his "collusion with the Russians" even after seven months of investigation.

"I am being investigated for firing the FBI Director (James Comey) by the man who told me to fire the FBI Director! Witch Hunt," Trump said in an early morning tweet.

Trump's apparent confirmation that he is under probe for obstruction of justice by special counsel appointed to probe alleged Russian meddling in the 2016 presidential polls came a day after a Washington Post report pointed to such an investigation.

The probe by special counsel Robert Mueller, a former FBI chief, "marks a major turning point in the nearly year-old FBI investigation, which until recently focused on Russian meddling during the presidential campaign and on whether there was any coordination between the Trump campaign and the Kremlin," The Washington Post had reported.

Trump did not respond to shouted questions from the media as he was leaving the White House lawns to board Marine One for the Joint Andres Air Force Base for his day-long trip to Florida.

"Despite the phony Witch Hunt going on in America, the economic and jobs numbers are great. Regulations way down, jobs and enthusiasm way up!" said the US President in another tweet.

He accused the mainstream American media of allegedly spreading the wrong news.

"The Fake News Media hates when I use what has turned out to be my very powerful Social Media -- over 100 million people! I can go around them," Trump said.

In the past, he has frequently used the term 'Fake News Media' for outlets like The New York Times, The Washington Post and CNN.

"After 7 months of investigations and committee hearings about my 'collusion with the Russians', nobody has been able to show any proof. Sad!" Trump said in his first tweet of the day.

A day earlier, Trump had questioned why his general election rival Hillary Clinton was not being investigated.

"Why is that Hillary Clintons family and Dems dealings with Russia are not looked at, but my non-dealings are?" Trump questioned.

"Crooked Hillary destroyed phones with hammer, 'bleached' emails, and had husband meet with Attorney General days before she was cleared -- and they talk about obstruction?" he had said.

The Post reported that the obstruction of justice investigation of the president began days after James Comey was fired as the FBI Director on May 9.

Mueller is overseeing a host of investigations involving people who are or were in Trump's orbit, people familiar with the probe were quoted as saying.

Significantly, Preet Bharara, the India-born former top US federal prosecutor, earlier this week had said that there was enough evidence to begin an obstruction of justice case against Trump over his alleged interference in the Russia probe.

Trump has said he is "100 per cent" willing to testify under oath on Comey meetings and tell the FBI Special Consul Robert Muller that he never asked FBI to stop investigating.