Defense Secretary James Mattis James Norman MattisBiden courts veterans amid fallout from Trump military controversies Trump says he wanted to take out Syria's Assad but Mattis opposed it Gary Cohn: 'I haven't made up my mind' on vote for president in November MORE on Sunday said the roughly 28,000 U.S. troops based in South Korea are “not going anywhere.”

“It's not even a subject of the discussions,” Mattis told reporters aboard his plane while returning to Washington from the Shangri-la Dialogue in Singapore.

Mattis added that other countries, including Canada, “are even putting people in. They're talking about bringing in light infantry, other troops of their own to show ... that we're all standing together.”

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President Trump Donald John TrumpBiden on Trump's refusal to commit to peaceful transfer of power: 'What country are we in?' Romney: 'Unthinkable and unacceptable' to not commit to peaceful transition of power Two Louisville police officers shot amid Breonna Taylor grand jury protests MORE last month said a forced withdrawal from South Korea is “not on the table,” in upcoming talks with North Korean leader Kim Jong Un. He was responding to reports that he had asked Pentagon officials to look at options for reducing troop numbers on the Korean Peninsula.

Trump has said, however, a future withdrawal could be possible, due to the upkeep cost of U.S. forces.

Trump, who canceled his June 12 summit in a letter to Kim late last month, reversed course on Friday and said the summit is back on schedule.

Mattis — who met with his Japanese and South Korean counterparts while in Singapore to discuss upcoming denuclearization talks with North Korea — said officials should “anticipate, at best, a bumpy road to the negotiations.”

He later told reporters “all negotiations are bumpy,” pointing to the on-again summit still scheduled for next week.

“You remember how we were going crazy — oh, gosh it's off when a bad letter comes in. And immediately it's back on? Welcome to reality,” Mattis said.

“We have more experience obviously than a lot of people do working like this together. So yes, it's going to be bumpy, but nothing late-breaking or too difficult.”