President Trump on Friday refused to say whether he is considering a pardon for former national security adviser Michael Flynn, who pleaded guilty last month to perjury charges and is now said to be cooperating with the special counsel investigation.

"I don't want to talk about pardons for Michael Flynn yet," Trump told reporters shortly before departing the White House to deliver remarks at the FBI National Academy.

"Let's wait and see," he added. "I will say this: when you look at what's gone on with the FBI and this investigation, people are very frustrated."

Flynn turned himself in to federal investigators on Dec. 1, noting at the time that his decision to work with special counsel Robert Mueller was made "in the best interests of my family and of our country."

The former White House aide is the fourth person close to the president to be charged as part of Mueller's ongoing probe into possible collusion between Russian officials and Trump associates.

Former Trump campaign chairman Paul Manafort, his deputy Rick Gates, and foreign policy adviser George Papadopoulos have also been charged by Mueller.

Trump issued his first presidential pardon for former Maricopa County Sheriff Joe Arpaio in late August. Arpaio, a major supporter of the president's immigration agenda, had been found guilty of contempt of court and was awaiting sentencing at the time of his pardon.