The country warned it would retaliate with "crushing strikes" if enemies intruded on its territory.
Officials said it would void past clear disarmament statements between North and South Korea.
Threats from North Korea come after UN voted on fresh sanctions. A volatile combination, even if Kim Jong-un's rhetoric is mostly bluster.
Crowds of soldiers run beach to greet North Korean leader Kim Jong-un as he confirms the end of non-aggression pact with the South.