President Trump, on his way to the G-7 summit in Canada, said he hopes to reinstate Russia into the group. Russia was kicked out in 2014 for its approach to Crimea.

"Russia should be in this meeting. Why are we having a meeting without Russia being in the meeting? And I would recommend, and it's up to them, but Russia should be in the meeting, it should be a part of it," Mr. Trump said, adding that "they should let Russia come back in."

Mr. Trump said he has been Russia's "worst nightmare," but they should still be invited. Meanwhile, Mr. Trump has been attacking U.S. allies over what he calls unfair trade practices, eliciting critical responses from French President Emmanuel Macron and Canadian Prime Minister Justin Trudeau.

Get Breaking News Delivered to Your Inbox

"We are going to deal with the unfair trade practices. If you look what Canada, Mexico, the European Union, what all of them have been doing to us for many decades, we have to change it," Mr. Trump said Friday.

On Friday, the president also said he's considering a posthumous pardon for boxer Muhammad Ali, after pardoning other high-profile individuals or individuals at the behest of celebrities lately.

"The power to pardon is a beautiful thing," Mr. Trump said. The president also reiterated his belief that he has the authority to pardon himself, but that won't be necessary.

"Yes, I do have an absolute right to pardon myself, but I will never have to do it becauseI didn't do anything wrong," Mr. Trump told reporters Friday.

Early Friday morning, just before he was to depart for a summit of world leaders, Mr. Trump renewed his criticism of U.S. allies over trade, tweeting, "Looking forward to straightening out unfair Trade Deals with the G-7 countries. If it doesn't happen, we come out even better."

Looking forward to straightening out unfair Trade Deals with the G-7 countries. If it doesn’t happen, we come out even better! — Donald J. Trump (@realDonaldTrump) June 8, 2018

He also singled out the country hosting the summit with another tweet: "Canada charges the U.S. a 270% tariff on Dairy Products! They didn't tell you that, did they? Not fair to our farmers!"

Mr. Trump will leave Quebec before the summit ends, flying directly to Singapore for his highly anticipated meeting with North Korean leader Kim Jong Un. The change in plans was announced by the White House after Macron and Trudeau signaled they would use the summit to take a stance against new U.S. tariffs on steel and aluminum imports.

As he departed Friday, the president said he has been preparing for the summit with North Korea all his life. Mr. Trump on Thursday said he thinks attitude is more important than preparation, but said he feels well-prepared..

"I said I've been preparing all my life," Mr. Trump said Friday. "I always believe in preparation. But I have been preparing all my life," President Trump says. "These one-week preparations, they don't work. Just ask Hillary (Clinton) — what happened to her in the debates."