She condemned ongoing South Korea-US military drills as "reckless" and accused Seoul's peace gestures of concealing a "sinister intention" against Pyongyang.
Her remarks followed South Korean President Lee Jae Myung's recent efforts to restore a 2018 inter-Korean military agreement and rebuild trust.
Kim Jong Un also criticised the joint drills and pledged a rapid expansion of North Korea's nuclear forces.
North Korea has abandoned its long-standing goal of peaceful unification with South Korea, now designating it a permanent enemy.