FILE PHOTO: Former U.S. national security adviser Michael Flynn passes by members of the media as he departs after his sentencing was delayed at U.S. District Court in Washington, U.S., December 18, 2018. REUTERS/Joshua Roberts/File Photo

WASHINGTON (Reuters) - The U.S. House Intelligence Committee has sent a letter to Michael Flynn, a former national security adviser to President Donald Trump, demanding he testify before the panel on Sept. 25, a committee aide said on Monday.

In a letter sent on Friday and seen by Reuters, the committee said Flynn had failed to comply with its subpoena, served on June 12, or cooperate with its efforts to secure his compliance.

Attorneys for Flynn did not immediately respond to a request for comment.

Flynn, a retired U.S. Army lieutenant general, resigned as Trump’s national security adviser in February 2017. He pleaded guilty in December 2017 to lying to the FBI about conversations with Sergey Kislyak, who was Russia’s ambassador to the United States, about U.S. sanctions imposed on Moscow by President Barack Obama.

That plea came in connection with Special Counsel Robert Mueller’s probe into Russian interference in the 2016 U.S. election.

Flynn was one of the first people in Trump’s inner circle to be charged by Mueller’s prosecutors for lying to investigators. Flynn worked on Trump’s election campaign and the conversations with Kislyak took place between Trump’s November 2016 election victory and his inauguration in January 2017.