Donald Trump insists no 'proof' on 'collusion' with Russia in 2016 election

The Washington Post first reported Wednesday that Mueller's probe was widening to include looking into whether Trump obstructed justice.

President Donald Trump has told his Twitter followers they are witnessing the "single greatest witch hunt in American history".

He tweeted, "After 7 months of investigations & committee hearings about my "collusion with the Russians", nobody has been able to show any proof".

Aside for Mueller, Feinstein also noted how Trump previously received a memo from Rosenstein, containing the Deputy Attorney General's recommendation ahead of the Comey firing.

Mr Mueller is leading an Federal Bureau of Investigation inquiry into Russia's alleged meddling in the 2016 presidential election.

Comey, who had been overseeing the bureau's Russian Federation investigation, says he had previously assured Trump he was not personally under investigation. Trump fired Comey in May.

Apparently, this major uptick in the investigation into Flynn, efforts at Russian sabotage and White House collusion and now the obstruction-of-justice element by Trump himself came days after then-FBI Director Jim Comey was sacked by Trump.

The White House could assert executive privilege to prevent those officials from being questioned about conversations with the president, which could lead to battles in court, where judges have tended to hold that criminal investigations can override such privilege, the New York Times reports.

Comey testified in a Senate hearing last week that he was sacked "because of the Russian Federation investigation", taking Trump at his word after the president made similar remarks during a television interview.

Examining such possible charges will allow investigators to interview key administration figures including Attorney General Jeff Sessions, Deputy Attorney General Rosenstein and possibly Trump himself, said the source familiar with the Mueller investigation.

Mr Trump was criticised for the tweet by the senior Democrat on the Senate Judiciary Committee, Dianne Feinstein.

The report comes a day after Attorney General Jeff Sessions, a close aide of Trump, vehemently denied any role in Russia's alleged meddling in the 2016 USA polls and dismissed accusations that he discussed such an effort with Russian officials as an "appalling and detestable lie".

Gingrich's observations also came amid news reports of President Trump added another high-powered lawyer to his personal team and Vice President Mike Pence had hired his own personal lawyer.