Robert Mueller has spoken out for the first time about his investigation into Russian meddling in US politics, saying charging President Donald Trump was not an option available to him.

The US Special Counsel’s public statement reiterated that Mr Trump was not exonerated and has reignited demands for the president’s impeachment.

In a nine-minute statement, Mr Mueller said he did not have the legal authority to pursue criminal charges against Mr Trump because a president could not be charged while in office.

Mr Mueller told reporters it was “not an option we would consider”.

Mr Mueller said his two-year investigation did not clear Mr Trump of improper behaviour and pointed out there were other ways to hold presidents accountable.

“The Constitution requires a process other than the criminal justice system to formally accuse a sitting president of wrongdoing,” Mr Mueller said.

Mr Trump reacted to the news in usual form by tweeting his innocence and declaring “the case is closed”!

“Nothing changes from the Mueller Report. There was insufficient evidence and therefore, in our Country, a person is innocent,” Mr Trump tweeted.

Nothing changes from the Mueller Report. There was insufficient evidence and therefore, in our Country, a person is innocent. The case is closed! Thank you.

— Donald J. Trump (@realDonaldTrump) May 29, 2019

Democratic presidential candidates have ratcheted up the pressure on US House Speaker Nancy Pelosi to start the process of impeaching Mr Trump.

Ms Pelosi has resisted calls to move forward on impeachment, arguing it could damage Democrats politically in advance of the November 2020 presidential election.

On Wednesday, she released a statement saying Mr Trump was not “above the law” but made no reference to initiating proceedings.

After Mr Mueller spoke, US Senators Cory Booker of New Jersey and Kirsten Gillibrand of New York, who were previously more restrained, actively advocated impeachment for the first time.

“Robert Mueller’s statement makes it clear: Congress has a legal and moral obligation to begin impeachment proceedings immediately,” Mr Booker tweeted.

Senator Kamala Harris also called for impeachment proceedings in a Twitter post.

“What Robert Mueller basically did was return an impeachment referral,” she tweeted.

“We need to start impeachment proceedings. It’s our constitutional obligation.”

What Robert Mueller basically did was return an impeachment referral. Now it is up to Congress to hold this president accountable.

We need to start impeachment proceedings. It's our constitutional obligation.

— Kamala Harris (@KamalaHarris) May 29, 2019

A spokesman for former vice president Joe Biden said the president’s “flagrant, open attacks on the rule of law” were troubling but he stopped short of demanding impeachment.

“Vice president Biden agrees with Speaker Pelosi that no one would relish what would certainly be a divisive impeachment process, but that it may be unavoidable if this administration continues on its path,” the spokesman for Mr Biden said.

Democratic chairman of the House of Representatives Judiciary Committee Jerry Nadler said “all options were on the table and nothing should be ruled out”.

“Given that Special Counsel Mueller was unable to pursue criminal charges against the President, it falls to Congress to respond to the crimes, lies and other wrongdoing of President Trump – and we will do so,” Mr Nadler said.

A redacted version of Mr Mueller’s report was published in April, concluding that Russia repeatedly interfered in the 2016 election and that Mr Trump’s election campaign had multiple contacts with Russian officials.

However, it did not establish a criminal conspiracy with Moscow to win the White House.

Donald Trump’s spokeswoman Sarah Sanders defends the president after Robert Mueller’s public statement. Photo: AAP

Mr Mueller, who headed the FBI from 2001 to 2013, said he would not elaborate beyond what was contained in his 448-page report, signalling to Democrats that he was unlikely to provide them more ammunition for impeachment if he were to testify on Capitol Hill.

The House Judiciary Committee’s top Republican, Representative Doug Collins, said re-litigating Mr Mueller’s findings would only divide the country.

“It is time to move on from the investigation and start focusing on real solutions for the American people,” he said.

Only one Republican so far, Representative Justin Amash, has said Mr Trump has committed impeachable offences. “The ball is in our court, Congress,” he said on Twitter.

-with AAP