SEOUL, South Korea — A former high-ranking North Korean diplomat who defected told NBC News that there is "zero" chance that Kim Jong Un, the country's leader, would willingly give up nuclear weapons because they help to keep him in power.

But President Donald Trump must nonetheless use economic sanctions to pressure the North Korean leader into agreeing to denuclearization, the former diplomat said.

"Kim Jong Un believes strongly that nuclear weapons are the last resort he can rely on for his future rule in North Korea," said the defector, Thae Yong Ho, once the second-highest ranking official at the North Korean embassy in London, in an interview in Seoul on Saturday.

Trump, who was in the region for the G-20 summit, became the first sitting U.S. president to set foot into North Korea on Sunday after an extraordinary, last-minute meeting with Kim at the demilitarized zone separating North and South.

After lengthy private talks, Trump said the two countries agreed to relaunch stalled nuclear negotiations within weeks. He added that sanctions on North Korea would remain in place for now, but seemed to leave open the possibility that some could be removed during the talks.

Kim would like the existing economic sanctions against his country to be lifted as soon as possible, Thae said.

"But I want to emphasize that the current level of economic sanction is not enough to force Kim Jong Un to totally abandon" the country's nuclear program, he said.

"He is afraid of additional sanctions," the former diplomat added.

For that reason, Thae said the two should have a third summit, after the second one that was held in Hanoi in March fell apart with no deal.

"The point must be the continuation of forcing Kim Jong Un to abandon [the country's] nuclear weapons," Thae said.

Trump "should deliver very clearly to Kim Jong Un that unless he abandons his nuclear program, the U.S., together with the world, will continue to sanction and will continue even to add more sanctions" until North Korea starts "the process of the denuclearization."

Sunday's unprecedented encounter between the two leaders followed Trump's public offer to meet Kim, inviting him to shake hands “and say hello.”

But Thae cautioned that such a public invitation could serve as powerful propaganda tool domestically in North Korea for Kim, as it suggests "that it is not North Korea begging or demanding a meeting, but it is America that wanted to have that meeting," he said.

The former diplomat said he thinks Kim likes Trump because he is "quite different than all previous American presidents."

"All previous American presidents are so far have been very moral and they paid great attention on the America’s moral image," Thae said.

In contrast, he suggested that Trump had not, for example, responded strongly enough to the case of American college student Otto Warmbier, who suffered severe injuries during his 17 months of detention in North Korea and died shortly after his release in 2017.

Richard Engel reported from Seoul. Phil McCausland reported from New York.