The House Intelligence Committee is threatening to enforce its subpoena seeking documents and public testimony from President Donald Trump's former national security adviser Michael Flynn, if he does not meet an extended deadline later this month.

In a letter to Flynn's attorneys, panel chairman Rep. Adam Schiff, D-Calif., called Flynn's effort to comply with lawmakers' requests "plainly insufficient" and accused his lawyers of exhibiting "a troubling degree of unprofessionalism" in how they had approached negotiations with the committee.

According to Schiff, Flynn's attorney Sidney Powell refused for a week to "accept service" of the panel's initial subpoena seeking Flynn's testimony. Schiff also complained that Flynn's lawyers attempted to direct the committee to collect subpoenaed documents from other congressional panels or the Justice Department, on grounds the requirements were too taxing.

The panel demanded that Flynn turn over all subpoenaed documents and materials by Sept. 18 and appear for testimony the morning of Sept. 25.

Powell did not immediately respond to a request for comment.

Flynn is one of several figures close to Trump who has caught the attention of congressional investigators examining their interactions with Russian officials.

The retired three-star general, who served in Trump's administration and on his transition team, pleaded guilty in late 2017 to lying to federal investigators about his contacts with former Russian ambassador Sergey Kislyak and cooperated with special counsel Robert Mueller's investigation of Russian interference in the 2016 election. Flynn has yet to be sentenced after several delays, initially due to Flynn's continued cooperation with prosecutors and later due to clashes between prosecutors and Flynn's new legal team.

Schiff accused those lawyers of occasionally refusing to speak with panel staffers, and attempting to assert an overly broad claim to Fifth Amendment protections.

"If you intend to assert your Fifth Amendment right against self-incrimination, your counsel must engage with the Committee Staff to discuss the potential subjects of your testimony so that the Committee may determine whether you have a good faith basis to assert your Fifth Amendment right," Schiff wrote. "Until that occurs, you remain obligated to appear in person and testify before the Committee."