"If we don't unify, and if the Kim Jong-un regime doesn't collapse, we will die," he was quoted as saying. "The issue of unification must be discussed as a matter of life or death."

Pyongyang has reportedly completed preparations to conduct a sixth nuclear test.

Thae noted South Koreans' apparent complacency about the possibility of a nuclear strike.

"Think about Kim Jong-un's character," he was quoted as saying.

"He has executed his relatives. Would he hesitate to use (a nuclear weapon) against the South Korean people?"

The North Korean regime must be brought down by its own people with the help of outside information, he added.

Thae worked as a minister at the North Korean Embassy in London before he defected with his family to South Korea last year. (Yonhap)