Washington: Reveling in his pardon powers, US President Donald Trump says he's thinking "very seriously" about pardoning the late Muhammad Ali, whose conviction already was overturned by the Supreme Court in 1971.

It's one of "thousands" of cases the president's team is reviewing, he told reporters as he left the White House en route to a world leaders' summit in Canada. And Trump said he's thinking about soliciting recommendations about other cases to consider from pro football players and other athletes who have protested racial injustice by kneeling during the national anthem - a tactic Trump has fiercely denounced.

Trump said that, "instead of talk," he's "going to ask all of those people to recommend to me - because that's what they're protesting - people that they think were unfairly treated by the justice system."

"I'm going to ask them to recommend to me people that were unfairly treated, friends of theirs or people that they know about and I'm going to take a look at those applications," he said.

Trump has been on a clemency kick of late, using his near-limitless power to pardon a growing list that includes a former White House aide, a conservative commentator and a former sheriff convicted of violating a judge's orders who campaigned with Trump in 2016.