"This has probably rarely been done. It's unknown territory in the truest sense," he said. "But I really feel confident and I feel that Kim Jong-un wants to do something great for his people and he has that opportunity and he won't have that opportunity again. It's never going to be there again."

Trump has offered security assurances and economic incentives in exchange for the complete, verifiable and irreversible dismantlement of North Korea's weapons of mass destruction programs.

"It's a one-time shot and I think it's going to work out very well," he said, adding that he will probably know "within the first minute" whether Kim is serious about the negotiations.

"I think that very quickly I'll know whether or not something good is going to happen. I also think I'll know whether or not it will happen fast," he said.

Describing Kim as an "unknown personality," the president still held out hope that the young leader will "surprise on the upside."

"But I've said it many times ... it may not work out. There's a good chance it won't work out. There's probably an even better chance that it will take a period of time. It will be a process," Trump said. (Yonhap)