A judge has delayed the sentencing of Donald Trump's former national security adviser Michael Flynn after telling him: "You sold your country out."
Flynn pleaded guilty in December 2017 to lying to the FBI about his conversations with Sergei Kislyak, the-then Russian ambassador in Washington.
The White House has said in a statement after the hearing that it still "firmly believes" the FBI broke protocol when they "ambushed" Flynn.
The statement added that there is "certainly a concern" that the former national security adviser lied but it will "let the courts determine that".
Flynn's lawyers had requested the sentencing in Washington be postponed to allow him to continue co-operating with the investigation into alleged collusion between the Trump campaign and Russian agents.
District Judge Emmet Sullivan said Flynn's actions bordered on treason and threatened the decorated former Marine general with a stiff prison sentence.
Judge Sullivan said he could not "hide my disgust, my disdain" for the crime, but later clarified that he was "not suggesting" Flynn committed treason.
He said Flynn committed a "very serious offence" by lying to the FBI on the premises of the White House.
The judge continued: "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.
"Arguably, you sold your country out."
The judge later admitted that he was mistaken in his remark that Flynn had been an unregistered agent.
The retired lieutenant general did however operate as an undeclared lobbyist for Turkey while working on Mr Trump's election team.
Special counsel Robert Mueller, head of the Russia probe, had recommended Flynn should not serve prison time because of his "substantial" co-operation with the investigation.
The judge did not set a new date for sentencing but asked Mr Mueller's team and Flynn's attorney to give him a status report by March 13.
Flynn admitted on Tuesday that he knew lying to the FBI was a crime as he stood by his guilty plea.
In a filing before the hearing in December, a lawyer for Flynn noted that he was not warned during his FBI interview that lying would be a crime.
But an attorney said during a sentencing hearing on Friday that the former national security adviser was not entrapped.
Hours before Flynn's sentencing, Mr Trump tweeted: "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!"
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!
— Donald J. Trump (@realDonaldTrump) December 18, 2018
Before the hearing, the federal judge made sure Flynn had entered his guilty plea knowingly and voluntarily.
Flynn told investigators in January 2017 that he had not discussed US sanctions against Russia with the diplomat, when according to his plea agreement he had.
Lying to the FBI carries a statutory maximum sentence of five years in prison.
However, Flynn's plea agreement stated he is eligible for a sentence of between zero and six months and can ask the court not to impose a fine.
Prosecutors said Flynn had already provided most of the cooperation that he could to the Russia probe, but it was possible he might be able to help investigators further.
In a separate development on Tuesday, Flynn's former business partner Bijan Rafiekian pleaded not guilty after being charged with unregistered lobbying for Turkey.
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Prosecutors say the lobbying effort was aimed at having the United States extradite a Muslim cleric who lives in America.
Rafiekian's trial date is 11 February and Flynn is expected to testify in that case.