Transcript for Trump vows to 'totally destroy North Korea' at UN assembly

We turn next to president trump. The world first hearing his warning to North Korea, promising fire and fury, and today, before the united nations, in his first major speech there, he went even further. The north Korean delegation could be seen leaving and ABC's chief white house correspondent Jonathan Karl is at the U.N. Tonight. Reporter: He has spoken of fire and fury, but today, president trump went even further, raising the possibility of wiping North Korea off the map. The United States has great strength and patience, but if it is forced to depend itself or its allies, we will have no choice but to totally destroy North Korea. Reporter: Then the president directly referred to north Korean dictator Kim Jong-un by his new nickname. Rocket man. Rocket man is on a suicide mission for himself and for his regime. Reporter: The north Korean delegation had seats in the front row, but they left right before the president spoke. The Iranian delegation, however, stayed and listened, as he accused Iran of funding terrorism and lashed out at the nuclear agreement negotiated with president Obama and America's European allies. The Iran deal was one of the worst and most one-sided transactions the United States has ever entered into. Frankly, that deal is an embarrassment to the united States. Reporter: Still, he did not say the U.S. Would withdraw from the agreement. The president cast this moment as a turning point for the U.N. It is entirely up to us, whether we lift the world to new heights or let it fall into a valley of disrepair. If the righteous many do not confront the wicked few, then evil will triumph. Jon Karl with us live tonight from the U.N. And Jon, president trump threatening to totally destroy North Korea, if forced to. The north Korean delegation walking out, as you reported there. Any word from North Korea tonight? Reporter: The north Koreans made it clear today they left the hall to boycott the president's speech. As for Iran, the Iranian foreign minister said, quote, trump's ignorant hate speech belongs in medieval times. But of course, David, that was a speech meant to put Iran and North Korea on notice. Jon Karl, who has been at the U.N. All day for us.

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