Transcript for President Trump raises the stakes ahead of Singapore summit

And we begin tonight with the world watching that high-stakes summit. President trump now just days away from coming face to face with Kim Jong-un. President trump in the air right now, he boarded air force one later this morning. Pulled out of the g7 early. Leaing our allies angry. He left a last-minute wide-ranging news conference, his first in nearly 500 days, the president talking tough about our allies, while praising the north Korean dictator, saying hopefully we would have liked each other. And within the first minute of their meeting, president trump says he'll be able to tell if Kim is a reliable negotiator. The president is now calling this trip, quote, a mission of peace. Our chief global affairs correspondent Martha Raddatz leads our coverage in Singapore. Reporter: President trump on air force one tonight, heading to the historic summit. Before taking off, increasing the pressure, saying he'll be able to see if Kim Jong-un is serious about easing tensions in an instant. How long will it take? I think within the first minute. I'll know. How? Just, my touch, my feel, that's what I do. How long will it take to figure out whether or not they're serious? I said maybe in the first minute. Reporter: And if the north Korean dictator isn't serious, president trump says he'll walk out of the room. I think I'll know pretty quickly, whether or not in my opinion whether something positive will happen. And if I think it won't happen I don't want to waste my time. Reporter: Now their face to face just days away the art of the deal president is trying to manage expectations for his most important deal yet. Start a dialogue. As a deal person I have done very well. I'd like to accomplish more than that. But at a minimum, at least we'll have met each other and we will have seen each other, and hopefully we would have liked each other. Reporter: The president shutting down questions whether his meeting with dictator might amount to just a photo-op? Everyone said, the haters, they say, oh, you're giving him a meeting, give me a break, okay, there's nothing -- it's never been done before. It's never been done before. Reporter: The president calling the summit a mission of peace, but warned this historic opportunity won't come again. So, it's a one-time shot. I think it will work out very well. You heard the president there. One-time shot, he says. Martha joins us from Singapore. Martha, with both leaders arriving tomorrow, you're already seeing tough security measures in place? Reporter: Some incredible security measures here, Cecilia, as you can imagine for both leaders. When you think about the fact that just six months ago both these leaders were threatening to annihilate each other. They have come a long way. Martha leads us off, thank you And we'll have much more on this historic meeting. George Stephanopoulos hosts this "This week" from Singapore in the morning and in the evening, Dade Muir brings us this broadcast, "World news tonight" from the summit.

This transcript has been automatically generated and may not be 100% accurate.