A new phase has been entered in the sprawling investigation into Russia and US President Donald Trump with the charging of former Trump campaign chairman Paul Manafort and two other aides.

The move underscores the continuing threat special counsel Robert Mueller poses to the president.

Mr Trump immediately sought to distance himself after Paul Manafort and Rick Gates pleaded not guilty on Monday to a 12-count indictment alleging money laundering, conspiracy and other offences.

Paul Manafort, centre standing and Manafort's business associate, Rick Gates, in federal court in Washington(Dana Verkouteren via AP)

Another former aide was revealed to be co-operating with authorities after entering a guilty plea to lying to the FBI.

White House officials were publicly optimistic about Mr Mueller’s investigation wrapping up swiftly, but the probe is far from over and its reach still uncertain.

Mr Trump has become increasingly concerned that the Mueller probe could be moving beyond Russia to an investigation into his personal dealings, two people familiar with the president’s thinking said.

Mr Trump expressed irritation on Monday that he was being tarnished by his former aides.

Paul ManafortPaul Manafort (AP Photo/Alex Brandon)

In the hours after the indictment, the president angrily told one confidant that Mr Manafort had been a campaign “part-timer” who had only helped steer the convention and got too much credit for Mr Trump’s ability to hold on to the nomination, according to a person familiar with the private discussion.

Mr Trump dismissed the money-laundering charges against Mr Manafort as typical political corruption that did not reflect on his campaign, one of the people said.

The president also insisted that the charges predated Mr Manafort’s time on the campaign and that he should not be held responsible for any prior misdeeds by him.

Mr Trump used Twitter to argue that allegations against Mr Manafort were from “years ago” and asserted there was “NO COLLUSION” between his campaign and Russia.

But the indictment against Mr Manafort and Mr Gates details allegations stretching from 2006 to 2017.

Mr Trump’s insistence that there was no collusion between his campaign and Russia was complicated by the disclosure that campaign adviser George Papadopoulos was answering questions from prosecutors after admitting he lied about his unsuccessful attempts to broker a meeting between Mr Trump and Russian President Vladimir Putin.

The White House tried to play down the campaign role of Mr Papadopoulos, who Mr Trump named as a foreign policy adviser in March 2016, saying the aide’s attempts to earn assistance from Russian nationals were unauthorised.

White House press secretary Sarah Huckabee Sanders dismissed him as an unpaid “volunteer” and said “no activity was ever done in an official capacity on behalf of the campaign in that regard”.

Mr Mueller’s office revealed in a court filing that Mr Papadopoulos was now assisting the investigation as a “proactive co-operator”.

Mr Trump fumed in recent weeks that he believes Mr Mueller was taking an expansive view of his role and looking beyond the narrow definition of alleged collusion between the Trump campaign and Russian officials.