Transcript for Trump cancels summit, pointing to NK's 'anger and open hostility'

And we begin with those major developments on North Korea. President trump calling off the summit on its nuclear program, citing North Korea's, quote, open hostility. It came just hours after north Korea invited journalists to say what they say was the planned destruction of their nuclear testing site. These images just coming in. And on both sides, tough talk. President trump reminding north Korea of America's overwhelming nuclear might. So, the question, does today increase the risk of war? ABC's Jonathan Karl with the president's answer. Reporter: President trump warned ominously the collapse of the summit is bad news for the entire world. I believe that this is a tremendous setback for north Korea, and indeed, a setback for the world. Reporter: The president pulled out in a wistful letter to Kim Jong-un, addressing the north Korean dictator as "His excellency," writing, "Dear Mr. Chairman. We greatly appreciate your time, patient and effort." But he wrote, due to north Korea's "Tremendous anger and open hostility," "I feel it is inappropriate, at this time, to have this long-planned meeting." He also issued a dark warning -- "You talk about your nuclear capabilities, but ours are so massive and powerful that I pray to god they will never have to be used." I've spoken to general Mattis and the joint chiefs of staff, and our military, which is by far the most powerful anywhere in the world, is ready, if necessary. Reporter: I asked the president if that could mean war. Mr. President, does the breakdown of this summit raise the risk of war with north Korea? Well, we'll see what happens. Reporter: The breakdown comes after a mind boggling thaw in u.s./north Korean relations, including two meetings between secretary of state Mike Pompeo and Kim Jong-un. And the release of three American prisoners. The president even relished talk of a Nobel peace prize. Do you deserve the Nobel prize, do you think? Everyone thinks so, but I would never say it. Reporter: But North Korea expressed revulsion at demands by U.S. Officials that they give up their entire nuclear program before getting anything in return. President trump tried to show some flexibility. I don't think I want to totally commit myself. But all in one would be a lot better. Reporter: But after vice president pence suggested dire consequences if Kim doesn't give up his nukes, North Korea lashed out, calling pence a, quote, political dummy. But in his letter, president trump told Kim, quote, "I felt a wonderful dialogue was building up between you and me." You said you had a wonderful dialogue with Kim Jong-un? We have a wonderful dialogue. We have a wonderful -- there's been a very good working relationship. Reporter: Today, North Korea made another overture, blowing up a nuclear test site in the presence of western journalists. And the president is still holding out hope, writing to Kim, "If you change your mind, please do not hesitate to call me or write." Do you expect him to call you, Mr. President? Do you expect him to call you? Jon Karl joins us from the white house. You were there in the room when president trump announced the summit was off. Is that still realistic? Reporter: No, that's not going to happen, Tom. White house officials made that clear after the president spoke. But there is no doubt the president would like to see this summit get back on track at some point, but it won't be on June 12th. Jon, thank you. Next to that other major story we're following, breaking

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