Former U.S. National Security Adviser Michael Flynn (R, Front) leaves the federal court following his plea hearing in Washington D.C., the United States, on Dec. 1, 2017. (Xinhua/Ting Shen)

Flynn took a combative turn against the government after Mueller concluded the investigation, moving to withdraw the guilty plea in January as his legal team claimed he was entrapped by the FBI agents.

The Department of Justice in May sought to dismiss the Flynn case, arguing that Flynn's lies could not be proved in court and were not material to the FBI investigation in early 2017 concerning the Russian interference. The DOJ's move drew intensifying questions about politicization in the prosecutorial process.

Sullivan refused to immediately grant the DOJ's request to drop the case, instead setting up a process to examine the department's reasoning.

Flynn, whose less-than-one-month tenure between January and February 2017 was the shortest in U.S. history for a national security adviser, was the second Trump associate convicted in the Russia probe. He was fired by Trump after pleading guilty.

Roger Stone, Trump's longtime confidant and GOP operative, was convicted of lying to Congress and witness tampering, but was commuted by the president in July, just days before he was due to report to prison.