President Trump concluded his time at the G-7 summit in Canada on Saturday by warning American allies against crossing the U.S. on trade. Speaking ahead of his planned departure for the North Korea summit in Singapore, he also called the anticipated meeting a "one-time shot" for Kim Jong Un to make North Korea "great."

Mr. Trump, who has sparred with other world leaders on Twitter over trade in recent days, warned other nations against retaliating against U.S.-imposed tariffs on steel and aluminum imports, and even warned that the U.S. might stop dealing with nations that fail to treat the U.S. fairly.

"If they retaliate, they're making a mistake," Mr. Trump said in a last-minute press availability ahead of his departure.

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Mr. Trump has made it clear he believes U.S. allies have taken advantage of the U.S. on trade for far too long, blasting them for what he believes are economically damaging trade deficits. On Saturday, the president said the U.S. is like the "piggy bank that everybody's robbing."

"At the top of the list was the issue of trade, very important subject because the Untied States has been taken advantage of for decades and decades and we can't do that anymore," Mr. Trump said.

Mr. Trump claimed he would prefer to have no tariffs among countries across the board, and no subsidies — "That's the way you learn at the Wharton School of Finance," he said.

The president also attacked U.S. reporters on foreign soil, attempting to discredit CNN reporters as "fake" people.

"You look at our farmers for 15 years the graph is going just like this down. Our farmers have been hurt our workers have been hurt our companies have moved out and moved to Mexico and other countries including Canada," Mr. Trump said. "Now we're going to fix that situation and if it's not fixed we're not going to deal with these countries the relationship I've had is great so you can tell that to your fake friends at CNN."

The president also declared the upcoming Singapore summit a "one-time shot" for Kim, as the world waits and watches for the outcome of the Tuesday meeting. Mr. Trump, asked what his objective for the summit is, didn't give specifics.

"I have a clear objective, but I have to say" that such situations are always "spur of the moment," he said.

Mr. Trump kicked off the second day of the G-7 summit in Canada in a similar fashion to how he arrived the day before: late. As the G7 summit breakfast on gender inequality began, there was an empty chair where Mr. Trump was supposed to be sitting, according to the White House press pool.

Still, Canadian Prime Minister Justin Trudeau, with whom the president has sparred on Twitter, began his opening remarks, followed by Canadian Ambassador to France Isabelle Hudon, at which point Mr. Trump arrived. French President Emmanuel Macron winked as Mr. Trump entered the room, according to the press pool.

President Donald Trump arrives late for a Gender Equality Advisory Council breakfast during the G-7 summit, Saturday, June 9, 2018, in Charlevoix, Canada. From left, German Chancellor Angela Merkel, IMF Managing Director Christine Lagarde, Trump, Christine Whitecross, and Winnie Byanyima. Evan Vucci / AP

On Friday, Mr. Trump left the White House residence behind schedule and stopped to speak with reporters for an extended period of time, forcing him to postpone his scheduled meeting with Macron until later in the day.—

Despite those tensions, the president claimed his relationship with other ally nations at the summit is a "10."

"I would say that the level of relationship is a 10," he claimed.

But even as he blasts ally nations for their trading practices — saying he blames past U.S. leaders more than them — Mr. Trump reiterated his desire that Russia come back to the negotiating table. Russia was kicked out of the group in 2014 over its approach to Crimea.

"Some people like the idea of bringing Russia back in," Mr. Trump said, without saying who said that.

"I think it would be an asset to have Russia back in," he added.

Mr. Trump is leaving the G-7 summit early, skipping discussions on the environment and climate change to head to Singapore. The president is expected to depart Canada en route to Singapore just before noon Saturday.

— CBS News' Arthur Jones, and Parita Desai, contributed to this report