Donald Trump has accused Barack Obama of embedding an FBI agent to spy on his presidential campaign.
The US president said it would be “bigger than Watergate” if his claims, which came in an early morning tweet, prove to be true.
His comments came after an earlier tweet in which he described the special counsel probe into possible coordination between the Trump campaign and Russia as "the greatest witch hunt in American history".
"Wow, word seems to be coming out that the Obama FBI `SPIED ON THE TRUMP CAMPAIGN WITH AN EMBEDDED INFORMANT,"' Trump said on Thursday.
"Andrew McCarthy says, `There's probably no doubt that they had at least one confidential informant in the campaign.' If so, this is bigger than Watergate!"
Mr McCarthy, a contributing editor at the National Review, wrote an article published last week headlined "Did the FBI Have a Spy in the Trump Campaign?"
Of special counsel Robert Mueller's investigation, Mr Trump said: "Congratulations America, we are now into the second year of the greatest Witch Hunt in American History... and there is still No Collusion and No Obstruction.
"The only Collusion was that done by Democrats who were unable to win an Election despite the spending of far more money!"
Mr Mueller, a former FBI director, was appointed to take over the Justice Department's investigation a year ago today.
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Mr Trump went on to say: “Despite the disgusting, illegal and unwarranted Witch Hunt, we have had the most successful first 17 month Administration in U.S. history - by far!
“Sorry to the Fake News Media and “Haters,” but that’s the way it is!”
Despite Mr Trump’s claims, the special counsel's office has charged 19 people – including four Trump campaign advisers – and three Russian companies.
Mr Trump's former national security adviser, Michael Flynn, and his deputy campaign chairman, Rick Gates, have pleaded guilty and are now co-operating with the probe.
Meanwhile, Trump's attorney Rudy Giuliani has been encouraging Mr Mueller's team to wrap up the investigation now that it has reached the one-year mark.
He said Wednesday that the probe "is not good for the American people, and the special counsel's office doesn't seem to have that sort of understanding that they're interfering with things that are much bigger than them."