Former Special Prosecutor Robert Mueller testifies before Congress on July 24, 2019, in Washington, DC.

Former special counsel Robert Mueller on Wednesday testified that President Donald Trump's praise for WikiLeaks during the 2016 campaign gave a "boost" to alleged "illegal activity."

WikiLeaks, the document disclosure advocacy group, had dumped troves of private emails from prominent Democrats, including the Democratic National Committee and Hillary Clinton's campaign chairman, in the final months of the 2016 presidential election.

The federal government alleges those accounts were hacked by Russian intelligence and passed along by WikiLeaks. The president had called the document dumps a "treasure trove" and declared "I love WikiLeaks!" at the time.

Mueller, in testimony before the House Intelligence Committee, was asked by Rep. Mike Quigley, D-Ill., to respond to Trump's positive comments about WikiLeaks.

"Well, problematic is an understatement in terms of what it displays, in terms of giving some hope, or some boost, to what is and should be illegal activity," Mueller said.