Flynn's actions during the transition 'were lawful': Trump

US President Donald Trump says he had to fire his former national security adviser because he lied to the FBI but defended Michael Flynn's actions as 'lawful'.

Donald Trump, left, and Michael Flynn. Source: Getty

US President Donald Trump says the actions of his former national security adviser Michael Flynn after the 2016 election were "lawful".

"I had to fire General Flynn because he lied to the Vice President and the FBI. He has pled guilty to those lies. It is a shame because his actions during the transition were lawful. There was nothing to hide!" Trump said in a tweet on Saturday.

He told reporters there was "absolutely no collusion" between his campaign and Russia in his first comment on the guilty plea.

"What has been shown is no collusion, no collusion," Trump told reporters as he departed the White House for fund-raising events in New York. "There's been absolutely no collusion, so we're very happy."

Flynn was the first member of Trump's administration to plead guilty to a crime uncovered by Mueller's wide-ranging investigation into Russian attempts to influence the 2016 US presidential election and possible collusion by Trump aides.

As part of his plea on Friday, Flynn agreed to co-operate with the investigation.

The retired US Army lieutenant general admitted in a Washington court that he lied to FBI investigators about his discussions last December with Russia's then-ambassador to the US, Sergei Kislyak.

In what appeared to be moves undermining the policies of outgoing president Barack Obama, the pair discussed US sanctions on Russia, and Flynn asked Kislyak to help delay a United Nations vote seen as damaging to Israel, according to prosecutors.

Flynn also was told by a "very senior member" of Trump's transition team to contact Russia and other foreign governments to try to influence them ahead of the vote, the prosecutors said.

Sources told Reuters the "very senior" transition official was Jared Kushner, Trump's son-in-law and senior adviser. Kushner's lawyer did not respond to multiple requests for comment.

Flynn, a former Defense Intelligence Agency director, only held his position as Trump's national security adviser for 24 days. He was forced to resign after he was found to have misled Vice President Mike Pence about his discussions with Kislyak.