Kim said the constitution should be amended to educate North Koreans that South Korea is a "primary foe and invariable principal enemy" and define the North's territory as separate from the South.
"We don't want war but we have no intention of avoiding it," Kim was quoted as saying by KCNA.
South Korean President Yoon Suk Yeol, at a cabinet meeting, said Pyongyang was being "anti-national" for calling the South a hostile country.
Analysts have said North Korea's foreign ministry could take over relations with Seoul, and potentially help justify the use of nuclear weapons against the South in a future war.
Ruediger Frank, professor of East Asian Economy and Society at the University of Vienna, said Kim's new policies "will trigger a cascade of changes across inter-Korean relations and regional dynamics".
Persons:
Hyunsu Yim, Kim Jong Un, Kim, KCNA, Yoon Suk, Ruediger Frank, Frank, Josh Smith, Lisa Shumaker, Jonathan Oatis
Organizations:
Supreme, Assembly, North, East Asian, Society, University of Vienna, U.S
Locations:
Hyunsu Yim SEOUL, Korea, North, Seoul, North Koreans, South Korea, KCNA . North Korea, South, Pyongyang