North Korea on Sunday threatened a "merciless military response" to the South Korea-U.S. Ulchi Freedom Guardian exercise that opened Monday.

The General Staff of the Korean People's Army said in a statement carried by the official Korean Central News Agency that the exercise is a nuclear war drill and practice for a future invasion of North Korea.

The North defined the drill as a substantial threat of war by the South, saying that "the war game commotion indicates real action aiming for full-blown military aggression."

It said Pyongyang’s military response will be the “worst-ever punishment any nation has ever experienced.”