US president Donald Trump has lashed out furiously amid mounting impeachment pressure, ranting and firing off a series of angry tweets about the so-called “witch hunt” against him.

A day after Robert Mueller broke his silence on an investigation into Russia’s meddling in the 2016 US presidential campaign, Mr Trump launched a scathing personal broadside against the US special counsel.

Mr Trump attacked Mr Mueller’s character, accused him of bias and described the investigation team as “some of the worst human beings on earth”.

He insinuated Mr Mueller had sour grapes about not being appointed FBI chief, tweeting that he had gone to the Oval Office “seeking to be named the Director of the FBI”.

“I told him NO. The next day he was named Special Counsel – A total Conflict of Interest. NICE!”

Robert Mueller came to the Oval Office (along with other potential candidates) seeking to be named the Director of the FBI. He had already been in that position for 12 years, I told him NO. The next day he was named Special Counsel – A total Conflict of Interest. NICE!

— Donald J. Trump (@realDonaldTrump) May 30, 2019

Mr Trump had earlier reacted with a single tweet to Mr Mueller’s public statement on Wednesday about the Russia investigation but came out swinging a day later on Thursday, seemingly rattled by the growing pressure from Democrats for impeachment.

During a press conference on the White House Lawn, Mr Trump angrily rejected any notion he had anything to do with Russia interfering to hurt Hillary Clinton’s presidential campaign, falsely claiming he had been exonerated.

But the wording of a Trump tweet seemed to infer Russia had indeed played a role in aiding his ascendance to the top job, saying “I had nothing to do with Russia helping me to get elected”.

Russia, Russia, Russia! That’s all you heard at the beginning of this Witch Hunt Hoax…And now Russia has disappeared because I had nothing to do with Russia helping me to get elected. It was a crime that didn’t exist. So now the Dems and their partner, the Fake News Media,…..

— Donald J. Trump (@realDonaldTrump) May 30, 2019

Mr Trump has scornfully rejected “disgusting” talk among Democrats about launching an impeachment inquiry against him after Mr Mueller said on Wednesday that his report on Russian collusion did not clear the president of obstruction of justice.

Mr Mueller indicated it was up to Congress to decide whether he should be impeached.

The statement fueled an increase in calls from Democratic lawmakers for impeachment proceedings, and US House of Representatives Speaker Nancy Pelosi, reacting to Mr Mueller’s statement, said “nothing is off the table”.

US Special Counsel Robert Mueller makes his first public statement since an investigation into Russian interference in the American presidential election in 2016. Photo: AAP

In response to the growing impeachment talk, Mr Trump tweeted it was the “Greatest Presidential Harassment in history”.

When asked by reporters if he expected to be impeached he said: “I don’t see how … it’s a dirty, filthy, disgusting word … it’s a giant presidential harassment,” he said, citing the US Constitution’s language that a president can be charged with “high crimes and misdemeanors.”

“There was no crime. There was no misdemeanor,” he said.

Mr Trump heaped scorn on Mr Mueller, calling him “totally conflicted” in part because he had wanted to head the FBI.

He said Mr Mueller was a close friend of former FBI Director James Comey, who Mr Trump fired weeks after taking office in early 2017.

“I think Mueller is a true ‘never Trumper,'” Mr Trump said.

-with AAP