He said Kim believes that with a nuclear-tipped missile, "then he can prevent that kind of humanitarian intervention".
"Before any military action is taken, I think it is necessary to meet Kim Jong-un at least once to understand his thinking and to try to convince him that he would be destroyed if he continues his current direction".
"I strongly believe if we educate the North Korean population we can change North Korea", he said.
Chinese President Xi Jinping replied to North Korean leader Kim Jong Un's congratulatory message on China's recently completed Communist Party Congress, saying he hopes to promote ties between the two countries, North Korea's official news agency KCNA said.
The testimony from Thae Yong Ho, former deputy chief of mission at the North Korean Embassy in London, underscored the high risk in using military force against North Korea. China has sought to defuse tensions between the United States and North Korea while also resisting moves that could lead to the collapse of Kim's regime.
He said he supported the Trump administration's policy of exerting "maximum pressure" on Pyongyang, but said it should be coupled with "maximum engagement" of North Korea's leadership and efforts to end the isolation of the country's 24 million people.
Thae said the US and South Korea would win a war after a preventive military strike, but there would be a "human sacrifice". "And we should tell Kim Jong Un that if Kim Jong Un stops this process and gives up his ambition of [intercontinental ballistic missiles] and nuclear development, American is ready to help Kim Jong Un build its economy and make North Korea a prosperous country".
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Thae Yong-Ho speaks at the Center for Strategic and International Studies on October 31.
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Thae, who is making his first visit to Washington since his defection previous year, said the USA and allied South Korea would win a war after a preventive military strike on the North, but there would be a "human sacrifice" inflicted on the South from the "tens of thousands" of artillery guns and short-range missiles the North has at the frontier.
North Korea has called Thae "human scum" and accused him of embezzling government money and committing other crimes.
North Koreans "don't care about state propaganda but increasingly watch illegally imported South Korean movies and dramas", weakening the regime's hold.
"We should start from basic concepts of human rights and freedom", Thae said, not railing against the Kim dictatorship and communism.