A federal court in Washington released audio of a voicemail that one of President Donald Trump's lawyers left for a potential cooperator with special counsel Robert Mueller's investigation.

The audio clip, less than two minutes in length, is of a November 2017 phone call from the Trump attorney, John Dowd, to a lawyer for former National Security Adviser Michael Flynn, who was poised to cooperate with Mueller in the days before his guilty plea.

It is a rare primary document underpinning one of the 10 possible instances of obstruction of justice by Trump and his associates that Mueller laid out in his report on Russian election interference.

Excerpts of the voicemail appear in Mueller's 448-page report. The transcript was released in full last week by request of U.S. District Judge Emmet Sullivan in Washington.

Dowd, in the voicemail, asked Flynn's lawyer for a "heads up" about whether Flynn intended to enter a cooperation deal with Mueller's' team. A warning would be necessary for the sake of "protecting all our interests," Dowd said, adding that any information implicating the president could lead to "national security issues." He also reminded the general's lawyer of the president's "feelings" toward Flynn and said "that still remains."

Flynn pleading guilty to lying to investigators about his contacts with the former Russian ambassador to the U.S., Sergey Kislyak, and awaits sentencing in Sullivan's court.