Special counsel Robert Mueller urged a judge to reject arguments made by attorneys for President Donald Trump’s former national security adviser Michael Flynn ahead of his sentencing next week, writing Friday in a court filing that, “the seriousness of the defendant’s offense cannot be called into question.”

“The Court should reject his attempt to minimize it,” Mueller wrote in a reply to a sentencing memo filed Tuesday by attorneys for Flynn. "Nothing about the way the interview was arranged or conducted caused the defendant to make false statements to the FBI on January 24."

In a memo filed Tuesday, Flynn’s attorneys said that unlike other defendants in the Russia probe, he wasn't warned in advance that it was a crime to lie to the FBI. The filing stopped short of saying FBI agents were looking to trap Flynn, but it did highlight that they went into the interview with a plan not to correct him if he strayed from what they knew of his calls with Russian Ambassador Sergey Kislyak.

Mueller said Friday that Flynn “chose to make false statements about his communications with the Russian ambassador weeks before the FBI interview, when he lied about that topic to the media, the incoming Vice President, and other members of the Presidential Transition Team.”

Flynn resigned from his post in the Trump administration in February 2017, after serving just 24 days in office. White House officials said at the time that Flynn misled Vice President Mike Pence about conversations he had with Kislyak.

He pleaded guilty in December 2017 to lying to the FBI about his contacts with Russian officials and agreed to fully cooperate with Mueller’s team.

Flynn’s attorneys asked a judge to sentence him to probation and community service in lieu of a prison sentence in the 178-page memo filed Wednesday. Prosecutors for Mueller’s office last week asked that Flynn serve little to no jail time in light of his cooperation with authorities. Prosecutors said Flynn has met with investigators 19 times in relation to three separate investigations, including Mueller’s probe into Russian meddling in the 2016 presidential election.

Flynn is scheduled to be sentenced Dec. 18 in the U.S. District Court for the District of Columbia.

The Associated Press contributed to this report.