U.S. President Donald Trump on Sunday launched a verbal attack on Special Counsel Robert Mueller's investigation into allegations of Kremlin meddling in the 2016 presidential election, calling it a "rigged witch hunt."

"There is No Collusion! The Robert Mueller Rigged Witch Hunt, headed now by 17 (increased from 13, including an Obama White House lawyer) Angry Democrats, was started by a fraudulent Dossier, paid for by Crooked Hillary and the DNC. Therefore, the Witch Hunt is an illegal Scam!" Trump said in one of a series of scathing Twitter messages, mentioning the Democratic National Committee, his predecessor Barack Obama, and former Secretary of State Hillary Clinton.

"Is Robert Mueller ever going to release his conflicts of interest with respect to President Trump, including the fact that we had a very nasty & contentious business relationship, I turned him down to head the FBI (one day before appointment as S.C.) & Comey is his close friend?" Trump continued, referring to Mueller's appointment as special counsel in charge of the Russian probe.

The verbal assault is the latest Mueller bashing seen on Trump's Twitter timeline, where it is a recurring theme.

So far the Russian probe has not yet produced concrete evidence that Trump knowingly colluded with the Kremlin during his campaign in 2016. However, dozens of Russians as well as key members of Trump's campaign have been charged with various crimes.

It is widely believed that Michael Cohen, personal lawyer to Trump during his campaign, may be in a precarious position after a raid on his office earlier this year yielded documentary and video evidence that shed more light on the Trump campaign's alleged interactions with Russia.

Reports released by Congress also determined that Russia indeed meddled in U.S. politics in Trump's favor, but stopped short of accusing Trump as a co-conspirator.

Pressured by U.S. media, Trump has conceded that Russia meddled in U.S. politics, but vehemently denied any collusion on his part.