US President Donald Trump has expressed no concern about the guilty plea by his former national security adviser Michael Flynn.

The president’s remarks on Saturday morning are his first public reaction to the plea deal, in which Flynn is co-operating with special counsel Robert Mueller’s investigation.

The president was at pains to stress there is “no collusion” between his campaign and the Russians.

Three times, Mr Trump told reporters it has been shown that there is “no collusion”.

Flynn has pleaded guilty to lying to the FBI about his contacts with Russia.

Former Trump national security adviser Michael Flynn leaves federal court in WashingtonMichael Flynn leaves federal court in Washington (Susan Walsh/AP)

Mr Trump spoke as he left the White House to head to New York for fundraisers expected to raise millions of dollars.

Flynn, who vigorously campaigned at Mr Trump’s side and then served as his first national security adviser, said members of the president’s inner circle were intimately involved with – and at times directing – his contacts.

The retired general’s plea to a single felony count of false statements made him the first official of Mr Trump’s White House to be charged so far in the criminal investigation by Mr Mueller.

Robert Mueller is leading the investigation (J Scott Applewhite/AP)Robert Mueller is leading the investigation (J Scott Applewhite/AP)

And his action could be an ominous sign for a White House shadowed for the past year by investigations.

The move could turn Flynn into a potentially key government co-operator as prosecutors examine whether Mr Trump’s campaign and Russia worked together to influence the 2016 presidential election in the president’s favour.

Friday’s developments do not resolve the paramount question of possible Trump-Russia co-ordination in the campaign, but they do show that Flynn lied to the FBI about multiple conversations last December with the Russian ambassador to the US.

Court papers make clear that senior officials for Mr Trump’s transition were fully aware of Flynn’s outreach to Russian officials in the weeks before the inauguration.

The officials were not named in court papers, but people familiar with the case identified two of them to the Associated Press as Jared Kushner, the president’s son-in-law, and former deputy national security adviser KT McFarland, now up for an ambassadorship.

Donald Trump’s son-in-law Jared Kushner is said to be the “very senior transition official” referred to in court papers (Evan Vucci/AP)Donald Trump’s son-in-law Jared Kushner is said to be the ‘very senior transition official’ referred to in court papers (Evan Vucci/AP)

That revelation moves the Russia investigation deeper into the White House.

And, given the direct involvement of the transition team in Flynn’s calls with ambassador Sergey Kislyak, the plea also raises questions about the accuracy of repeated assertions by the administration that Flynn had misled Mike Pence and other officials when he denied having discussed sanctions with the diplomat.

Flynn stood quietly during his plea hearing except to answer brief questions from the judge.

He accepted responsibility for his actions in a written statement, though he said he had also been subjected to false accusations.

“My guilty plea and agreement to cooperate with the Special Counsel’s Office reflect a decision I made in the best interests of my family and of our country,” he said.

A former Defence Intelligence Agency chief, Flynn was a considerably more vocal surrogate for Mr Trump during the campaign, known for leading rally crowds in “lock her up” chants regarding Hillary Clinton’s use of a private email server.

Though prosecutors had also investigated Flynn for lobbying work on behalf of the Turkish government, the fact that he was permitted to plead guilty to just one count, and faces a guideline range of zero to six months in prison, suggest that prosecutors see him as a valuable tool in their investigation and are granting a degree of leniency in exchange for co-operation.

White House lawyer Ty Cobb sought to distance the plea from Mr Trump himself, saying: “Nothing about the guilty plea or the charge implicates anyone other than Flynn.”