A US judge has fiercely criticised President Donald Trump's former national security adviser Michael Flynn for lying to FBI agents, and delayed sentencing to ensure Mr Flynn cooperates fully with an investigation into possible collusion between Mr Trump's election campaign and Russia.

Key points: Former national security adviser Michael Flynn pleaded guilty to lying to the FBI

Former national security adviser Michael Flynn pleaded guilty to lying to the FBI His sentencing was delayed so he could cooperate in ongoing investigations into Russian collusion

His sentencing was delayed so he could cooperate in ongoing investigations into Russian collusion A Judge told Mr Flynn that: "Arguably, you sold your country out"

Mr Flynn pleaded guilty in December 2017 to lying to FBI agents about his conversations with Sergei Kislyak, Russia's ambassador in Washington at the time.

During Mr Flynn's sentencing hearing, US District Judge Emmet Sullivan told the retired army lieutenant general that his behaviour was abhorrent, noting that Mr Flynn had also lied to White House officials, who in turn lied to the public.

"Arguably, you sold your country out," Judge Sullivan said.

"I'm not hiding my disgust, my disdain for this criminal offence."

President Donald Trump's former national security advisor Michael Flynn, his attorney Robert Kelner and his wife Lori Andrade arrive at federal court in Washington. ( AP: Carolyn Kaster )

At one point, the judge asked prosecutors if Mr Flynn could have been charged with treason, although he later said he had not been suggesting such a charge was warranted.

Russian ambassador to the US Sergey Kislyak. ( AP: Cliff Owen )

He said he had read additional facts about Mr Flynn's behaviour that have not been made public.

The judge initially noted that Mr Flynn was operating as an undeclared lobbyist for Turkey even as he worked on Mr Trump's election team and prepared to be his national security adviser.

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But after the hearing, Judge Sullivan appeared to walk back some of his criticisms, clarifying that Mr Flynn was no longer working as an unregistered foreign agent during his short stint as national security adviser.

He said he could not promise that he would not eventually sentence Mr Flynn to serve prison time.

Judge Sullivan appeared ready to sentence Mr Flynn to prison but then gave him the option of a delay in his sentencing so that he could fully cooperate with any pending investigations. Mr Flynn accepted that offer.

Special Counsel Robert Mueller, leading the investigation, had asked the judge not to sentence Mr Flynn to prison because he had already provided "substantial" cooperation to prosecutors over the course of many interviews.

Judge Sullivan did not set a new date for sentencing but asked Mr Mueller's team and Mr Flynn's attorney to give him a status report by March 13.

Lying to the FBI carries a statutory maximum sentence of five years in prison.

Mr Flynn's plea agreement stated that he was eligible for a sentence of between zero and six months.

The charges Mr Flynn has pleaded guilty to carry a sentence of up to five years in prison. ( AP: Carolyn Kaster )

Trump backs Flynn on Twitter

Mr Flynn served as a high-profile adviser to Mr Trump's election campaign team, and at the Republican Party's national convention in 2016 he led Mr Trump's supporters in cries of "Lock her up!" directed against Democratic candidate Hillary Clinton.

He became national security adviser when Mr Trump took office in January 2017 but lasted only 24 days before being ousted.

Donald Trump, pictured with Vladimir Putin last year, has denied allegations of Russian collusion with his election campaign. ( Reuters: Sputnik/Mikhail Klimentyev/Kremlin )

He told FBI investigators on January 24, 2017, that he had not discussed with Mr Kislyak US sanctions imposed on Russia by the administration of Mr Trump's Democratic predecessor Barack Obama, when in fact he had, according to his plea agreement.

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Mr Trump has said he fired Mr Flynn because he also lied to Vice-President Mike Pence about the contacts with Mr Kislyak.

But Mr Trump has said Mr Flynn did not break the law and has voiced support for him, raising speculation the Republican President might pardon him if he was sentenced.

"Good luck today in court to General Michael Flynn. Will be interesting to see what he has to say, despite tremendous pressure being put on him, about Russian Collusion in our great and, obviously, highly successful political campaign. There was no Collusion!" Mr Trump wrote on Twitter on Tuesday morning.

Sorry, this video has expired White House press secretary Sarah Sanders criticises FBI

White House press secretary Sarah Sanders defended Mr Trump's tweet, saying it was "perfectly acceptable" for him to make a positive comment while the court deliberated.

She criticised the FBI's conduct, alleging they encouraged Mr Flynn not to be represented by White House counsel when he was questioned.

"The FBI broke standard protocol in the way that they came in and ambushed General Flynn," she said.

She added the agency "thought they could get away with it" because they were dealing with the Trump administration.

Sorry, this video has expired Michael Flynn lashed by judge as sentencing delayed

Mr Mueller's investigation into Russia's role in the 2016 election and whether Mr Trump has unlawfully sought to obstruct the probe has cast a shadow over his presidency.

Several former Trump aides have pleaded guilty in Mr Mueller's probe, but Mr Flynn was the first former official of Mr Trump's White House to plead guilty.

Mr Mueller has also charged a series of Russian individuals and entities.

Robert Mueller's probe into Russia's role in the 2016 election has cast a shadow over the Trump presidency. ( AP: Charles Dharapak )

Mr Trump has called the investigation a "witch hunt" and has denied collusion with Moscow.

Russia has denied meddling in the election, contrary to the conclusion of US intelligence agencies that have said Moscow used hacking and propaganda to try to sow discord in the United States and boost Mr Trump's chances against Ms Clinton.

Several protesters were at the courthouse on Tuesday as Flynn arrived, along with a large inflatable rat.

Reuters