Former National Security Advisor General Michael Flynn leaves after the delay in his sentencing hearing at US District Court in Washington, DC, December 18, 2018.

The House Intelligence Committee on Thursday announced that it had issued subpoenas to Trump's former national security advisor Michael Flynn as well as his top campaign aide Rick Gates for documents and testimony related to Russian interference in the 2016 election.

"As part of our oversight work, the House Intelligence Committee is continuing to examine the deep counterintelligence concerns raised in Special Counsel Mueller's report, and that requires speaking directly with the fact witnesses," said Rep. Adam Schiff, D-Calif., the committee's chairman. "Both Michael Flynn and Rick Gates were critical witnesses for Special Counsel Mueller's investigation, but so far have refused to cooperate fully with Congress."

It is not clear whether either man will comply with the subpoeanas. Attorneys for Flynn and Gates did not immediately respond to requests for comment. The subpoenas call for Flynn and Gates to provide documents by June 26 and sworn testimony on July 10.

Gates, a close associate of former Trump campaign chairman Paul Manafort, served as the Trump 2016 campaign's deputy campaign manager. He was indicted in 2017 as part of special counsel Robert Mueller's Russia probe and pleaded guilty last year to a conspiracy charge and to lying to investigators. He has not yet been sentenced.

Flynn is awaiting sentencing for his own lying charge brought by Mueller. In recent days, though, Flynn swapped lawyers and hired Sidney Powell, who has advocated for the retired United States Army lieutenant general to withdraw his guilty plea.

In a letter to Flynn accompanying the subpoena, Schiff suggested that cooperating with the committee could ease Flynn's potential sentence.

"It would further underscore to your sentencing judge that 'you've done everything you possibly can for the United States of America,' not just the Department of Justice," Schiff wrote.