US special counsel Robert Mueller has said he is resigning from his role and returning to private life.

He has led the investigation into alleged Russian collusion in the 2016 presidential election.

In a rare press briefing at the US Department of Justice on Wednesday, Mr Mueller said that charging US President Donald Trump was not an option his office could consider.

Mr Mueller told reporters he could not have charged Mr Trump with an offence while he was sitting as president, because of longstanding Justice Department protocol.

He said "it would be unfair" to potentially accuse someone of a crime when they could not stand trial to defend themselves.

Announcing the closure of the office and his return to private life, Mr Mueller said: "If we had confidence that the president did not commit a crime we would have said so.

"We did not make a conclusion."

However, he defended the investigation, saying it was important for prosecutors to investigate while evidence was fresh in witnesses' minds.

He also said it allowed co-conspirators to potentially be brought to justice.

Opening his statement, Mr Mueller reiterated the basis of the investigation, which came about because of evidence of a "concerted attack" on the US which was "designed and timed to interfere with the elections and damage a presidential candidate".

He refused to enter further debate about whether the individual Russian citizens who posed as Americans online to influence the election were guilty or not.

He said the basis on the investigation meant the team also had to probe efforts to obstruct it.

He said: "When a subject lies or obstructs it strikes at the heart of the government."

In a response to the development, Mr Trump tweeted: "The case is closed! Thank you."

In a statement, White House Press Secretary Sarah Sanders said: "The Special Counsel has completed the investigation, closed his office, and has closed the case.

"Mr Mueller explicitly said that he has nothing to add beyond the report, and therefore, does not plan to testify before Congress.

"The report was clear - there was no collusion, no conspiracy - and the Department of Justice confirmed there was no obstruction.

"Special Counsel Mueller also stated that Attorney-General Barr acted in good faith in his handling of the report.

"After two years, the Special Counsel is moving on with his life, and everyone else should do the same."

The US Justice Department released a redacted version of the report last month.

The 448-page, two-volume report set out multiple episodes in which Mr Trump directed others to influence the Russia probe.

Among those was the disclosure of how the president urged an aide to instigate the sacking of Mr Mueller himself.

Some of the redacted Mueller Report is seen in April 2019 | Image: USA TODAY Network/SIPA USA/PA Images

In June 2017 - a month after Mr Mueller's appointment to probe possible cooperation between Mr Trump's 2016 election campaign and Moscow - the president attempted to remove Mr Mueller from his position, the report said.

The lengthy document described how Mr Trump called White House lawyer Don McGahn and told him to call then-attorney general Jeff Sessions to say Mr Mueller "had conflicts of interest and must be removed".

"McGahn did not carry out the direction, however, deciding that he would resign rather than trigger what he regarded as a potential Saturday Night Massacre," the report added, referencing the firing of key officials during former president Richard Nixon's Watergate scandal.

Mr Mueller's work focused on Russian hacking and social media campaigns, possible Russian government links to - and contacts with - the Trump campaign, and potential obstruction of his investigatory efforts.

Mr Trump's efforts to influence the investigation "were mostly unsuccessful, but that is largely because the persons who surrounded the president declined to carry out orders or accede to his requests," Mr Mueller wrote.

Although he did not conclusively find that Mr Trump had committed criminal obstruction of justice, Mr Mueller did not exonerate the president on the question either.

He found no evidence of collusion between Mr Trump's campaign team and Russia - despite "numerous links" - but said the president's campaign "expected it would benefit electorally from information stolen and released through Russian efforts", referring to hacked Democrat e-mails.

Additional reporting: IRN