The exercise is in response to joint drills that have been held by South Korea, Japan and the US this week - which Pyongyang repeatedly denounces as a rehearsal for war.

State media set out how the North envisions a potential war in unusual detail.

The drill rehearsed repelling a sudden invasion, striking its neighbour with nuclear weapons, then sweeping in to occupy territory.

In the scenario, "simultaneous super-intense strikes" also targeted military command centres, ports, operational airfields and other targets in South Korea.

Hours after the US deployed B-1B bombers for allied air drills on Wednesday, North Korea also fired two short-range ballistic missiles into the sea.

That late-night launch followed two unsuccessful attempts to put a spy satellite into orbit.

Kim Jong Un's secretive state has been condemned by Japanese Prime Minister Fumio Kishida, who told reporters: "These conducts pose threats to peace and stability of not only our country, but of the region and international community, and cannot be tolerated."

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The Japanese government has stressed it will intercept North Korean missiles if they fly over its territory.

Kim has been urging his military to become increasingly ready - describing leaders of the US, South Korea and Japan as "gang bosses" who increase the risk of nuclear war in the region.