Thae Yong-ho, who recently compared Kim to the Roman emperor Nero, said North Korea is headed down a dangerous path as it seeks recognition as a nuclear weapons state like "India or Pakistan," Voice of America reported Tuesday.
"The Kim Jong Un regime will never give up nuclear weapons," Thae said.
The high-profile defector added North Korea could "fall apart" if the regime decides to conduct a major nuclear test at its Punggye-ri nuclear site, where Pyongyang conducts tests of weapons of mass destruction.
Recent satellite images show tunnel digging continues at the site, which lies below the 800-meter Mount Mantap.
According to 38 North, the activity indicates North Korea may be prepared to conduct additional underground nuclear tests.
Thae said Tuesday the site is located on a road that connects Pyongyang to Hamgyong Province, and that "roads and railways that go up north pass by the nuclear test site."
"If a large explosion takes place and the area becomes contaminated with radiation while Pyongyang loses control of the border region of North Hamgyong Province, mass defections could take place," Thae said.
A nuclear failure in a "small country like North Korea" could lead to disaster, the defector said, adding China and the international community must be aware of the danger.
Thae also said Kim Jong Un is trying to achieve parity with South Korea's relatively more powerful military with his nuclear weapons program.
In an annual ranking of militaries around the world, South Korea troops ranked the 11th most powerful in 2016, while North Korea's military strength ranked 25th in the survey by website Global Firepower.