Trump admits, briefly, that Russia boosted his election

US President Donald Trump acknowledged in a tweet that Russia helped him get elected, before backtracking in comments to White House reporters. (AFP/Jim WATSON)

"Russia, Russia, Russia! That's all you heard at the beginning of this Witch Hunt Hoax," Trump tweeted, in an outburst against Special Counsel Mueller's suggestion that Congress impeach him for obstructing the two-year investigation.

"And now Russia has disappeared because I had nothing to do with Russia helping me to get elected," Trump said.

It appeared to be the first time that Trump accepted claims by US intelligence chiefs that Russian government meddling aided his stunning upset victory over Democrat Hillary Clinton.

Although Trump did not remove the tweet, he shortly afterward sought to walk back the admission, telling reporters as he left on a trip to Colorado that Russia "if anything, helped the other side," or Clinton.

"Russia did not get me elected. You know who got me elected? I got me elected," he said.

"Russia didn't help me at all."

Trump has stridently rejected any suggestion his victory was illegitimate ever since US intelligence chiefs announced in January 2017 that Moscow interfered heavily in the election, hacking computers and manipulating social media largely to damage Clinton and boost Trump's campaign.

His outburst came a day after Mueller - in his first public comments on the investigation he was named to lead in May 2017 - said it had established there "were multiple, systematic efforts to interfere in our election."

Special Counsel Robert Mueller speaks on the investigation into Russian meddling in the 2016 presidential election at the US Justice Department. (AFP/MANDEL NGAN)

"Russian intelligence officers who were part of the Russian military launched a concerted attack on our political system," he said, with their hacking "designed and timed ... to damage a presidential candidate."

TRUMP: MUELLER IS 'CONFLICTED'

Mueller also reiterated that the investigation found evidence of attempts to obstruct his investigation by Trump.

"If we had confidence that the president clearly did not commit a crime, we would have said that," he said.

But he said he was blocked from charging the president by Justice Department regulations and indicated that it is up to Congress to launch impeachment proceedings to determine if Trump committed a crime.

"When a subject of an investigation obstructs that investigation or lies to investigators, it strikes at the core of the government's effort to find the truth and hold wrongdoers accountable," Mueller said.

Trump in response slammed Mueller as conflicted and said the investigation had produced no evidence against him.

"Robert Mueller should have never been chosen," to lead the probe, he told reporters on Thursday.

"I think he is a total conflicted person. I think Mueller is a true never-Trumper."

"He is somebody that dislikes Donald Trump. He is somebody that didn't get a job that he requested."

Asked about a growing clamor in Democratic ranks for the launch of impeachment proceedings against him, Trump called it "presidential harassment."

"To me it's a dirty word , the word impeach, it's a dirty, filthy, disgusting word, and it had nothing to do with me," he said.

"There was no high crime and there was no misdemeanour," he said, referring to the grounds defined in the US constitution for removing a president.

"So how do you impeach based on that?"

Democratic leaders have remained cautious about taking the politically fraught step to remove the president, with elections less than 18 months away.

Nancy Pelosi, the Democratic leader in Congress, said in a statement that Congress would step up investigations, while studiously avoiding the word impeachment.

US House Speaker and Democratic Congressional leader Nancy Pelosi is resisting pressure to launch impeachment proceedings against President Donald Trump. (AFP/MANDEL NGAN)

"The Congress holds sacred its constitutional responsibility to investigate and hold the President accountable for his abuse of power," she said.

"The American people must have the truth."

But several Democratic presidential hopefuls are pressing for Congress to start impeachment proceedings.

"If he were anyone other than president of the United States, he would be in handcuffs and indicted," said Senator Elizabeth Warren.

"What Robert Mueller basically did was return an impeachment referral. Now it is up to Congress to hold this president accountable," Senator Kamala Harris said on Wednesday.