"You were an unregistered agent of a foreign country while serving as the national security adviser to the President. Arguably, this undermines everything this flag over here stands for," he said.

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

Sullivan appeared unimpressed that Flynn's lawyer had suggested in an earlier court filing that Flynn was not warned during an FBI interview that lying would be a crime.

Flynn said on Tuesday he knew lying to the FBI was a crime as he stood by his guilty plea. An attorney for Flynn also told the court that Flynn was not entrapped.

Flynn, who served as national security adviser for only a few weeks, was to be the first White House official sentenced in Mueller's investigation into possible coordination between Russia and the Trump campaign.

The hearing took place amid escalating legal peril for Trump, who was implicated by federal prosecutors in New York this month in hush-money payments to cover up extramarital affairs. Nearly a half-dozen former aides and advisers - including Flynn - have pleaded guilty or agreed to cooperate with prosecutors.

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Sullivan told Flynn that he would take into account his extensive cooperation with the government, which includes 19 meetings with investigators as well as a 33-year military career that included service in Iraq and Afghanistan.

But he also said he was forced to weigh other factors, too, including Flynn's decision as national security adviser to lie to the FBI on the premises of the White House.

The judge warned Flynn that he may not get full credit for his cooperation if he was sentenced as scheduled on Tuesday since prosecutors said there's a possibility that they may call on him to assist with other investigations down the road.

He gave Flynn a chance to talk it over with his lawyers, and the court went into a brief recess. When they returned, both sides agreed to delay the sentencing.

In another dramatic moment in Tuesday's hearing, Sullivan asked whether Flynn's crimes rose "to the level of treason" before later retracting his question and apologising.

"I feel terrible about that," Sullivan said, adding he had never presided over a treason case. Prosecutor Brandon Van Grack said that "the government has no reason to believe that the defendant committed treason".

Trump had earlier signalled his intense interest in the case by tweeting "good luck" to Flynn hours before the sentencing.